Hardee Richard Cox. MOISTURE FLUX IN A FIELD OF MAIZE.
(Under the direction of Richard A. Stephenson) Department
of Geography and Planning, July 1986.
The purpose of this study is to understand the
phenomenon of stemflow as it occurs in a field of maize.
The approach used is to first develop a conceptual stemflow
model and then to observe and collect data for application
to the model. Stemflow can be defined as water that is
collected on appendages of vegetation and subsequently
directed in its path toward the Earth's surface to impact
the soil in the immediate vicinity of the intercepting
plant.
Stemflow as a variable in interception studies has not
been widely studied. There are several reasons for this,
such as time, expense and difficulty of measuring
stemflow. As a consequence, there is a common conceptual
model of precipitation in which stemflow is an
underestimated and/or hidden variable. Data gathered from
four years of record aided in formulating a new conceptual
model with the water variables appropriately arranged as to
the soil surface, their area impact and each other. The
new stemflow model was used as a basis, in every rainfall
event, for portraying the moisture variables and the
amounts measured of each
^, Hó
X
S Íd
MOISTURE FLUX IN A FIELD OF MAIZE
A Thesis
Presented to
the Faculty of the Department of Geography and Planning
East Carolina University
In Partial Fulfillment
of the Requirement for the Degree
Master of Arts in Geography
by
Hardee Richard Cox
June 1986
ECU LIBRAR^
MOISTURE FLUX IN A FIELD OF MAIZE
by
Hardee Richard Cox
APPROVED BY:
DEAN OF THE GRADUATE SCHOOL
ACKNOWLEDGEMENTS
I would like to thank Mr. T. W. Bowers and family for
allowing me the use of their property, and Ms. Cynthia
James for her help in producing the final copies. A hearty
thanks goes to the faculty of the Geography and Planning
Department for their encouragement and suggestions. There
are several people to whom I owe a special thanks: to my
parents for their enduring support; to Dr. Don Steila for
his enthusiasm and ideas on the subject; to Dr. Richard
S tephenson for his friendship and prompting to complete
this; and to my wife Angie for her patience and caring
throughout this study; not to mention all her typing. Thank
you all.
TABLE OF CONTENTS
PAGE
LIST OF FIGURES V
CHAPTER 1. INTRODUCTION 1
Research Objectives 1
Justification 2
Background Review 3
Present State of Knowledge 7
Stemflow Defined 9
CHAPTER II. THEORETICAL FRAMEWORK 12
Precipitation Distribution in Maize 12
Conceptual Modeling 18
CHAPTER III. RESEARCH TEST SITE DESCRIPTION 24
Soils 25
Instruments and Equipment Used 29
Equipment Siting and Use 29
The 1981 Corn Crop 34
CHAPTER IV. OBSERVATIONS OF PRECIPITATION PATTERNS . . 37
First Rainfall Event 38
Second Rainfall Event 40
Third Rainfall Event 42
Fourth Rainfall Event 44
Fifth Rainfall Event 44
Sixth Rainfall Event 47
Seventh Rainfall Event 47
Eighth Rainfall Event 50
Ninth Rainfall Event 50
Summary 53
CHAPTER V. CONCLUSION 54
BIBLIOGRAPHY 57
LIST OF FIGURES
FIGURE PAGE
1. COMMON PRECIPITATION MODEL 14
2. ACTUAL MODEL 17
3. IDEALIZED MODEL 19
4. TEST SITE LOCATION 26
5. TEST PLOTS AND SOILS 27
6. EQUIPMENT SITING 30
7. STEMFLOW CATCHMENT SYSTEM 32
8. PRECIPITATION PATTERN, JUNE 7, 1981 39
9. PRECIPITATION PATTERN, JUNE 19, 1981 41
10. PRECIPITATION PATTERN, JULY 2, 1981 43
11. PRECIPITATION PATTERN, JULY 6, 1981 45
12. PRECIPITATION PATTERN, JULY 8, 1981 46
13. PRECIPITATION PATTERN, JULY 16, 1981 48
14. PRECIPITATION PATTERN, JULY 18, 1981 49
15. PRECIPITATION PATTERN, JULY 22 , 1981 51
16. PRECIPITATION PATTERN, JULY 30, 1981 52
CHAPTER I
INTRODUCTION
The purpose of this research is to understand the
phenomenon of stemflow as it occurs in a field of maize.
Steraflow is a relatively unstudied variable, and many
questions should be ansviered. The first step is to develop
a conceptual stemflow model. The second step is to observe
and collect data. These two steps comprise the scope of
the research.
Research Objectives
The research objectives are; (1) to identify how plant
physiognomy is related to stemflow volume relative to
precipitation amounts, and (2) how stemflow relates to the
spatial variation in recharging soil moisture. Stemflow
represents water (precipitation and/or dew) that is
collected on the appendages of vegetation and subsequently
directed in its path toward the Earth's surface to impact
the soil in the immediate vicinity of the intercepting
plant. This capability of vegetation, a function of plant
structure, causes uneven soil wetting. When rainfall
events are of low intensity and brief duration, many plants
can effectively concentrate meager precipitation amounts at
the soil surface in close proximity to the area of thickest
root development. In corn, this capability is especially
significant in providing the crop with available soil water
during the period prior to maturation. Evidence indicates
2
that leaf characteristics in corn such as leaf area index
and canopy cover increases through crop maturity.
Thereafter, leaves droop downv/ard, reducing water
collection efficiency.
Justification
The application of this research would be to use the
information contained herein for: (1) water conservation in
agriculture, and (2) regional and national water resource
development programs. With the supply of water relatively
constant, consumptive use of water by industry, urban
complexes, and agriculture is increasing. The result is a
conflict in user's priority of water. In response, several
states have, or are now considering, legislation that
limits consumer use of water from either surface or
groundwater sources. This action is designed to prevent,
or at least alleviate, critical water shortages that have
recently occurred, even in humid regions, and that are
predicted to become more severe in the future. In
particular, these impending water use restrictions are
being structured during a time when agriculturists are
drastically increasing irrigated acreages (Snead, 1981).
Resolution of the impending "water demand crisis" will be
largely based upon more stringent and wiser use of our
currently available resources. (Anonymous, 1984)
This study will add to the general body of
microclimatic information. The data base developed from
3
1981 observations in addition to data from three previous
crop seasons are the only information known to exist for
stemflow of small plants. Also, it will serve as a
refinement of existing évapotranspiration models for
agricultural purposes. The study will also provide a basis
for the further sophistication of crop moisture modeling
and could have significance in scheduling irrigation.
Total moisture flux in plant communities has only been
studied by forest meteorologists or climatologists who deal
with large and widely spaced plants. Simplicity of
monitoring water movement within habitats dominated by
semi-permanent, large plants has made forest environments
ideal sites for monitoring such aspects of moisture flux as
infiltration or throughfall, detention, and stemflow. The
literature reveals that none of these variables have been
adequately measured in microclimatic field studies, that
is, within the lower ten feet of the atmosphere. Most of
our agricultural economy is based in this realm of boundary
layer air.
Background Review
Assessment of the role of water in agricultural
climatology has been largely relegated to its significance
in évapotranspiration processes. Consequently, most
moisture model research relative to field crops has been
unidirectional, concerned primarily with movement or
potential movement of water from the root zone to the
4
atmosphere. This, of course, solely relates to water
research in microclimatic studies and does not refer to
water erosion, flood control, or general research of
unsaturated soil water flow. When soil moisture is at
optimal levels, the concept of field crops operating as
hydraulic pumps in response only to energy availability is
viable. Optim.um soil moisture status, however, is not the
norm under field conditions. Even in humid climates, soil
water reserves frequently fall below fifty percent
potential storage. Monitoring of soil moisture status
beneath field corn in eastern North Carolina, during 1977
and 1981, has confirmed these findings. Average soil
moisture status of less than one-half potential storage
occurred during seventy one percent of the 1977 crop season
(Steila and Wall, 1978). In 1981, these conditions were
present fifty eight percent of the time. Such periods of
deficit moisture status represent times of drying of the
rhyzosphere. It is at these times that farm managers
attempt to improve crop yield through water applications.
Current research which focuses on stemflow phenomenon
and évapotranspiration modeling reveals that the role of
plant structure as an element in affecting recharge of soil
water via stemflow has not been studied in sufficient
detail. This lack of detailed research results in
erroneous conclusions about stemflow. Such conclusions
generally evolve from four distinct problems; (1) stemflow
5
data are scarce; (2) data are only available for forest
species; (3) data are seldom correleted with soil moisture
recharge information; and (4) available data are
incompatible with other moisture flux indices with which
they are compared.
Patrie and Helvey (1965) point out that stemflow
monitoring is expensive, tedious, and time consuming.
Consequently, it is not surprising that this phenomenon has
received minimal research attention. Obtaining stemflow
data in a forest environment requires the installation of
semi-permanent, gutterlike collars on sample trees. The
gutter is sealed to the tree to collect stemflow and divert
it into retaining tanks for later measurement. The absence
of stemflow data for agricultural crops undoubtedly rests
in a compounding of the collection problem when dealing
with a nonpermanent, small plant.
Steila (1976) has resolved this problem by devising a
serviceable, polyethylene collar for use with small plants
such as corn. The collar has been tested throughout three
growing seasons prior to this study and on a 1981 corn
crop.
Seldom have forest stemflow data been correlated with
soil moisture status. The few studies that have discussed
this relationship have expressed it in qualitative terms.
Even so, these studies demonstrate the impact that stemflow
may have upon redistribution of precipitation and its
6
capability of concentrating intercepted water into the root
zone of plants. The following serves as an example:
Uneven soil moistening may result from
interception of rain by plants. Part of the rain
reaches the soil by flowing down the stem of the
plant. Some plants such as mulga (Acacia aneura)
in Australia intercept virtually all the rain
falling on them. In this situation, the soil is
moistened deeply around the stem, but remains
extremely dry under the canopy of the mulga, while
between plants the rain mostly falls directly on
the soil. Thus, we have in one large pedon three
contrasting regimes; in one, the soil is deeply
moistened; in the second, which surrounds the
first,the soil receives virtually no moisture; in
the third, the soil is intermittently moist and
dry (U.S. Department of Agriculture, 1975).
The generally accepted format for expressing stemflow
data is incompatible with other moisture supply indices.
Previous researchers have obtained volumetric stemflow data
from sample trees (or sample plots of trees) and simply
divided its volume by the intercepting canopy area to
establish an area/depth water equivalent that was
subsequently compared with gross precipitation. The
outcome was a determination of the stemflow's significance
to plants. This approach frequently yields an
insignificant measure of water reaching the surface via
stemflow. However, this method does not give the real
picture, particularly in small plants, such as corn.
Stemflow, as demonstrated in the previous quote, is not
uniformly distributed over the soil surface. Rather, it
impacts the soil surface where it is available for
infiltration or in the immediate vicinity of the plant's
7
contact with the ground. To be correctly interpreted, that
is, relative to gross precipitation and throughfall which
are area impact derived values, stemflow should be defined
as an area/depth measure determined by an impact zone
circumferential to the intercepting plant (Steila, 1981).
Present State of Knowledge
A search of recent literature reveals few references to
substantive stemflow research. Of these studies none dealt
with the stemflow phenomenon in agriculture crops. The
only known stemflow research in agriculture has been at
East Carolina University. Steila and Vvall (1977 and 1978 )
made preliminary studies in the summers of 1976 and 1977.
Steila continued and updated this research during the
summer of 1979. In 1981 research was conducted and
constitutes the data base for this study with supplemental
information from the three previous years.
Stemflow is most evident in trees. During a rainfall
event, water can be seen running down the trunks. For this
reason, earliest references to stemflow were by foresters.
Foresters were also the first to monitor stemflow as a
variable within interception studies.
Patrie and Helvey (1965) proposed that foresters
monitor stemflow on randomly selected plots, by using
"narrow collars sealed to the trunks of sample trees to
divert downflowing water into storage containers." This
approach is accepted as the most feasible method for
determining stemflow volume and has been universally
applied. Of the references cited that related to tree
stemflow, all used a narrow collar for intercepting and
channelizing stemflow into a retaining collector. In a
fashion similar to the technique implemented by foresters,
stemflow was monitored in randomly selected corn fields,
utilizing a polyethalene collar, during the summers of
1976, 1977 , 1979, and 1981. The collars collected stemflow
and funneled it into a sealed cylinder from which total
volume could be determined and subsequently compared with
characteristics of both rainfall events and soil moisture
status change.
Analytical results of stemflow data usually vary in
accordance with the investigator's interests. Patrie and
Helvey (1965) related total seasonal and annual stemflow
values to the gross precipitation parameter. Their
evaluation involved converting volumetric stemflow to an to
an area/depth value by dividing its cubic unit equivalent
by canopy intercepting area of sampled trees. Their
conclusion was that stemflow comprised an insignificant
portion of gross precipitation (five percent or less). In
Australia while working with a native shrub called mulga
(Acacia aneura), Pressland (1973) arrived at an opposite
conclusion. After monitoring stemflow in mulga, it was
determined that virtually one hundred percent of the
rainfall was intercepted by the plant and diverted to the
9
surface, adjacent to its base, as stemflow. This study
demonstrated stemflow efficiency in concentrating
precipitation within the soil as potentially available
transpirable water. Since a large percent of the soil
surface beneath mulga canopies is normally dry, the study
also illustrated the water conservation potential of
plants. In the same line of thought as Pressland (1973),
Chang (1968) noted that as much as fifty percent of
évapotranspiration may be associated with water evaporated
from wet soil surfaces. Dry soil surfaces do not lose
water through evaporation.
Other stemflow studies have focused upon precipitation
thresholds for initiation of the phenomenon in various
forest stands. Ford's (1978) research on canopy structure
and precipitation intensity, and studies underway at the
Hydraulic Research Station, Wallingford, England, (Steila,
1983) pointedly indicate that forest species attain maximum
stemflow efficiency prior to maturation, and then stemflow
efficiency tends to decrease. This same increase and
decrease of stemflow efficiency holds true for corn.
Stemflow Defined
Stemflow can be defined as water (precipitation and/or
dew) that is collected on appendages of vegetation and
subsequently directed in its path toward the Earth's
surface to impact the soil in the immediate vicinity of the
intercepting plant. It is commonly monitored by a
10
gutterlike collar attached to the plant and connected to a
retaining tank. Water collected in the retaining tank is
converted to an area/depth value by dividing its volume by
area of the plant's stemflow impaction upon the soil
surface, referred to as stemflow impact area (SIA). The
SIA is an area delimited on one side by the basal perimeter
of the intercepting plant and on the other side by a
circumferential outer boundary. The latter is
equidistantly spaced at one centimeter from the former
(Steila, 1981) .
Earlier interpretations of stemflow defined the
phenomenon as depth of plant redirected water uniformly
distributed over the soil surface throughout an area
bounded by the intercepting plant's canopy. Pressland's
(1973) research on mulga stemflow supports the foregoing
statement that earlier analytical methods were erroneous.
Stemflow is not uniformly distributed over the soil
surface. Earlier approaches do not represent the potential
of soil moisture recharge. Gross precipitation and
throughfall, for example, are monitored in free-catch
containers of constant intercept area, and expressed as
depth of water impacting the soil. To be equivalent, for
comparative purposes, stemflow needs also to be evaluated
in relation to the area wherein its water impacts the
surface. The one centimeter wide catchment area has been
utilized in corn stemflow evaluation. Although the
11
catchment area or Stella's SIA has been presented before
professional groups for evaluation, no negative response
has been received with regard to areal delimitations. Both
Dr. Stella and this researcher consider the SIA to be
subject to modification as continued research on the
stemflow phenomenon proceeds.
CHAPTER II
THEORETICAL FRAMEWORK
Precipitation Distribution in Maize
In a study of moisture flux in plants, there are two
separate yet related elements. The first, commonly known
as precipitation, deals with movement of atmospheric
moisture towards the earth. This encompasses the different
forms of precipitation and the differing modes of movement
of this water. Stemflow, plant detention, precipitation,
and throughfall, which is moisture that falls directly onto
the soil or has been in contact with plant appendages but
has fallen from them, are the modes of downward movem.ent of
water. The second element is évapotranspiration, that
being the movement of moisture from the soil and/or plant
upward into the atmosphere. It is the first element of
moisture movement, the earthward direction that is to be
presented in this section as it is found in a field of
maize.
A standard model of how a rainfall event deposits water
upon the surfaces below, both plant and soil, can be shown
by the following:
PPT = T + (S + PD) (1)
where :
PPT is gross precipitation,
T is throughfall.
S is stemflow, and
13
PD is plant detention.
This equation divides gross precipitation into two
categories with throughfall constituting one part, and all
the remaining amounts of water comprising the second part
in unit volume per unit area values (Figure 1).
There are two problems that arise by describing
precipitation distribution in this matter. First, by
grouping stemflow and plant detention together into a
single variable the model is overly simplified. One reason
for this grouping is associated with monitoring and/or
gathering raw data. Monitoring stemflow is time consuming
(Patrie and Helvey, 1965). This is compounded when working
with a non-permanent plant such as maize. During the
monitoring of the 1981 maize crops, the collars used to
catch stemflow had to be repaired every time any amount of
stemflow was recorded. Also, the collars were replaced
every third or fourth monitoring visit to allow for the
change in stalk size and/or shape of cross-section.
Another factor is that stemflow' has only recently been
observed scientifically and i s still a variable being
measured in agroclimatic studies. Plant detention has been
studied even less. so the lack of data has generated only a
simplistic precipitation distribution model.
The second problem is that by combining stemflow and
plant detention, the importance of stemflow in contributing
to the total amount of water available for plant use is
 
15
masked. This format of expressing stemflow is not
compatible with other moisture variables. Researchers that
have obtained stemflow data have divided its volume by the
area of the intercepting canopy to arrive at an area/depth
water value. In so doing, stemflow is thought to be
insignificant in contributing to available water. However,
stemflow is not evenly distributed on the soil surface
beneath the plant canopy, but is concentrated adjacent to
trunks or stems of plants, or as in maize, the stalk.
Since other moisture parameters are derived as area impact
values, stemflow values should be determined in the same
manner. This means that stemflow should equal a unit depth
value related to its soil surface impact area. To apply
this approach, the basic conceptual model of precipitation
needs to be modified. This refinement changes the
precipitation distribution model, and with respect to
maize, will more accurately portray the distribution of
precipitation.
There are two major modifications of the conceptual
model (Figure 1). First, stemflow and plant detention are
separated and treated as individual variables. The
equation has been reformulated to show this as:
PPT = T + S + PD (2)
where:
PPT is gross precipitation.
T is throughfall,
16
S is stemflow, and
PD is plant detention.
This permits stemflow evaluation relative to its
significance as a descrete variable. Also, plant detention
can be determined by reformulating equation (2) as:
PD = PPT - (T + S) (3)
so as to isolate it for analysis.
In the second adjustment, stemflow is treated as an
area/depth value, that is the depth of stemflow relative to
the unit area of the soil surface it impacts. These
changes produce a realistic model of how precipitation is
actually distributed in a field of corn (Figure 2). The
original model of precipitation distribution is from an
unpublished manuscript, based upon data collected for the
period 1976 - 1979 (Steila, 1982).
The unorthodox nature of the actual precipitation model
(see Figure 2) is a result of how the variables stemflow,
throughfall, and plant detention, are located or positioned
relative to a stalk of maize in a natural crop setting.
For example, examine the moisture pattern on and around a
stalk of maize after a brief shower. The soil surface is
moistened by water directly impacting it. Water is flowing
down the stalk from the blades to moisten the soil adjacent
to the stalk. On the extremities of the plant, water
neither moving nor yet evaporated is present. This plant
detained water is not in direct contact with the soil
ACTUAL MODEL
JUNE -i TO AUGUST 1
PLANT DETENTION 57.0m
94.7mm+C805.6mm+57.0mm3r:957.3mm UNIT VOLUME/UNIT AREA
1
FIGURE 2
18
surface, so in the model it is elevated above the other
moisture variables.
In an effort to include all flow the variables, yet
produce a simplified descriptive model, plant detention can
be graphically repositioned beneath throughfall and
stemflow. This arrangement simplifies the pictorial model
and retains an actual portrayal of each moisture variable.
In addition, data from the ten stemflow columns which are
observed in the field are combined to produce one column
encompassing an area equivalent to the ten columns.
Since the actual model presents variables as area/depth
values, another item must be added. Maize stalks occupy
soil surface space, wherein water interception cannot
occur. This area is represented as a zero area/depth
variable in the model. All of the foregoing are portrayed
in an idealized model of precipitation distribution (Figure
3). These modifications result in a model that allows
stemflow, throughfall, and plant detention to be compared
to each other in terms of volumetric amounts with respect
to their appropriate areas of impact.
Conceptual Modeling
There are four concepts concerning corn stemflow. They
are derived from observations gathered during the summer of
1981 and from three prior years of monitoring, 1976, 1977,
and 1979. The first concept states that soil moisture
recharge potential changes with respect to the variables'
IDEALIZED MODEL
JUNE 1 TO AUGUST 1
GROSS PRECIPITATION
94.7mm + 805.6mm + 57.Om;Ti = 957.3mm UNIT VOLUME/UNIT AREA
FIGURE 3
20
impact area in all directions away from the basal perimeter
of a maize plant. This statement differentiates
infiltrated soil water from potential absorbed soil water.
A conceptual model that expresses the relation between soil
moisture recharge potential and distance is;
R = Th. + SF . (4)
p 1 1
where ;
R is soil moisture recharge potential,
P
Th? is water available from throughfall impacting
the surface, and
SF. is water available from stemflow imipacting
the surface.
This model assumes that in a relatively level field with a
uniform stand of corn there will be a distinctive pattern
of natural sources of water reaching the surface. The
pattern will be columnar in the immediate vicinity of each
plant, with the remainder of the surface having a
relatively equitable receipt of moisture in lesser
amounts. Other factors such as structure, density of
plants and soil cover can also influence recharge.
Further, the seasonal soil moisture recharge tends to
decrease with distance from the basal perimeter of the
plants. This is expressed:
= VI /D (5)
sm 3
where ;
R is soil moisture recharge,
sm
W is water available for recharge, and
3
D is distance from basal perimeter of a plant.
21
There are two primary factors relating to actual soil
moisture recharge relative to distance from a corn plant:
(1) available impacting precipitation, and (2) soil
infiltration capacity. As stated previously, more water is
available adjacent to the plant than at any other point on
the surface due to the additional contribution of
stemflow. In addition, more water is infiltrated into the
soil adjacent to a corn plant. Another aspect of
infiltration potential relates to soil porosity. Water
flowing down the corn stalk repeatedly floods the soil
close to the plant's basal perimeter. This results in
colloidal size particles being subjected to suspension and
displaced away from the plant by floatation. The remaining
surface soil in the immediate vicinity of the stalk is
usually coarse textured relative to the soil surrounding
it, and is more capable of infiltrating water. This does
not suggest that all stemflow infiltrates the soil.
In addition, stemflow efficiency is related to the
stemflow/precipitation ratio and precipitation intensity.
This is modeled as:
SE = SP^/P. (6)
where :
SE is stemflow efficiency,
SP is the unit depth stemflow/unit depth
precipitation, and
P^ is precipitation intensity.
The most effective tunneling of water to the plant's soil
22
contact zone yields high ratios, which can be in excess of
3:1. Such conditions occur when precipitation events are
of low intensity and magnitude. VJhen a less than 12 hour
storm intensity generates more than 20 mm precipitation,
the ratio becomes relatively constant at 2.0 - 2.5:1
(Steila, 1982).
Finally, total water available for plant use (i.e.
transpiration) can exceed both gross precipitation and
throughfall indices. This is conceptualized as:
Wta > P (7)
and
W, > Th (8)
13.
where :
is total water available for
ta plant use,
P is gross precipitation, and
Th is throughfall.
Evapotranspiration models normally assume gross
precipitation to be representative of the maximum water
available for potential évapotranspiration demands. The
underlying theory of such models generally relates to zonal
climatic and soil conditions, and to a combination of
energy availability and water supply components for
determining water loss from a vegetative cover and its
associated soils. Although these models can reasonably
estimate long term évapotranspiration, they cannot
distinguish between water budget components such as
23
evaporation or transpiration. As a result, potentially
available plant water has normally been determined by gross
precipitation or throughfall. However, it has been
observed that in corn, during drought periods, low
intensity precipitation is directed via stemflow to
selected areas where plant roots are concentrated and that
wetting of potential evaporative soil surfaces is largely
eliminated. The significance of these findings is that for
specified rainfall events, the plant's available water can
be in excess of either of the two generally accepted
potential soil moisture recharge indices.
CHAPTER III
RESEARCH TEST SITE DESCRIPTION
Pitt County, North Carolina lies in the physiographic
province known as the Atlantic Coastal Plain. The
sediments comprising the area were deposited by both marine
and fluvial processes. Coastal Plain soils have formed on
unconsolidated sands, silts, and clays that have been
subjected to a weathering sequence encompassing 225,000 -
250,000 years (Steila, 1982). Of the more than thirty
different soil series found through Pitt County, seventy
four percent are grouped within the soil order Ultisols, an
order containing the most weathered soils in the
conterminous United States.
"The climate of Pitt County is influenced by elevation,
by distances from the Atlantic Ocean and the Pamlico Sound,
and by latitude and location of the county in the
continent" (Hardy, 1974). Temperature averages for the
year are 73° maximum daily and 50° minimum daily based
upon monthly averages. The growing season lasts from March
until November, or about 220 days.
Precipitation averages forty eight inches per year with
July being the wettest month and November, the driest.
"Thunderstorms account for a large part of the rainfall
received during the growing season" (Hardy, 1974). The
remainder of the year large low pressure storms influence
precipitation.
25
The location of this research project is in the
northern tip of Pitt County, two miles east of Bethel
(Figure 4). The area is drained by Grindle Creek which
flows southeast into the Tar River. Approximately fifty
percent of the area is under cultivation. Pasture and
woodland cover the remainder of the area surrounding the
test sites. Of the cultivated fields, many have an
extensive subsurface drainage system. Subsurface drainage,
using tiles in conjunction with open ditches, enable rapid
removal of water.
The individual test plots are scattered through two
adjoining fields (Figure 5). Plots one, two, and three are
in a field with tile drainage. Sites four and five are in
a field that does not have tile drainage. However, this
field has a greater elevation and more slope. A large open
drainage ditch separates the fields and connects to a
natural drainageway.
A standard weather shelter was positioned in an open
space adjoining the T.W. Bowers farmstead. The station
housed several meteorological instruments, including, a
Bacharach Recording Hygro-thermograph. The close proximity
of an inhabited house detered tampering and vandalism of
the instruments, thereby aiding in the accuracy of the
readings.
Soils
There are two soil types that have evolved in the test
FIGURE 4
FIGURE 5
28
area. The Exum series dominates the area with the Coxville
series occuring sporadically between individual Exum
polypedons. Both soils are fine sandy loams suitable for
cultivation. The study area is located between the inner
and outer portion of the Atlantic coastal plain. This
location results in a landscape which is partly composed of
fairly well preserved marine derived forms, and partly
fluvial forms related to subaerial erosion. The marine
terraces are vividly portrayed with relict beach strands
and shallow offshore flats. Much of the marine terraces
are poorly drained as the drainage system has, as yet, not
become mature or integrated. However, some areas have
succeeded in being drained, both naturally as well as
anthropically. This géomorphologie situation has allowed a
wide variation in topographic and drainage expression, and
in the soils as well.
There are seven soil associations derived from forty
two mapping units in Pitt County (USDA, 1974). The
Norfolk-Exum-Goldsboro soil association which includes
Coxville, is moderately well to well drained, although the
Coxville tends to be poorly drained, and located on the
Wicomico strand, and a small area of Penholloway strand.
This is the only true upland soil association in the county
(Stephenson, 1985). All five test plots are situated in
Exum soils (Figure 5). The Coxville soil series occupy
slightly lower topographic elevations with anthropic
drainageways.
29
Instruments and Equipment Used
Several instruments were used to monitor moisture
flux. The equipment consisted of:
(11)m(1)inim28umsmall cylindrical rain gauges(2) sheets of plastic for stemflow collars(3) tin cans to collect stemflow(4) a one square metering grid(5) a rod marked in inches for crop height(6) a gypsum block for soil moisture(7) a soil moisture resistance meter(8) a Hygro-thermograph(9) two thermometers(10) tape and other supplies as neededEach test plot had at least two collars and containersfor stemflow, three rain gauges for throughfall, and theburied gypsum blocks (Figure 6). Data was gathered foreach moisture variable and related items. They are: (1)stemflow, (2) throughfall, (3) gross precipitation, (4)soil moisture under the stalk, (5) soil moisture betweenthe rows, (6) crop height, (7) percent crop cover, (8)blade length, (9) blade width, (10) maximum temperature,temperature, (12) relative daily humidity, and
(13) average daily temperature. The maize crop in these
fields was observed from planting in April through
post-maturity in mid August.
Equipment Siting and Use
Gypsum blocks were implanted in all five test plots.
The blocks were in groups of three with two groups per test
plot. One group was centered under corn stalks at depths
of six, twelve, and eighteen inches below the ground
FIGURE 6
31
surface (Figure 6). The se cond group was implanted between
rows at the same depths. The i res leading to the blocks
were arranged and marked to indicate depth of each block
and to increase speed in monitoring. When the blocks were
implanted the soil was replaced keeping the texture and
structure as undisturbed as possible. Each test plot had a
similar monitoring system and equipment layout. Soil
moisture percentage was observed with a resistance meter at
each depth and then recorded. Each test plot was
positioned in the fields to avoid intense water run-off or
collection.
To monitor stemflow, polyethylene collars, designed by
Steila (1976), were fashioned and fitted around selected
stalks slightly above the support roots. A gallon can with
the top almost entirely covered was positioned underneath
the spout of the collar to catch stemflow (Figure 7). A
total of fourteen catchment units were constructed. Each
sample plot contained a minimum of at least two units.
After each rainfall event the stemflow collected in each
catchment container was emptied into a bottle calibrated in
cubic centimeters and the volumetric measurement recorded.
The collars and cans required constant maintenance and
repair throughout the growing season, particularly during
rapid stalk development prior to tasselling.
Field rain collectors were stationed throughout the
test area (Figure 5) at a mature canopy level to measure
FIGURE 7
33
gross precipitation. The same type of gauge was used to
collect throughfall, which is precipitation that directly
impacts the ground beneath the crop canopy. Each test plot
was equipped with three collectors for measuring
throughfall (Figure 6). These were randomly placed on the
ground in an effort to achieve accurate sampling.
A one meter square grid, divided into one hundred one
square decimeter sections, was positioned on the ground
covering a section of the number one test plot (Figure 6).
A rod could then be positioned perpendicular to the grid at
each decimeter corner. Where the rod touched a stalk or
blade, that point was considered covered by the crop. All
one hundred points were tested on each site visit to arrive
at a representative percentage of crop cover for the field.
A hygro-thermograph was placed in a U.S. Standard
Weather Shelter and subjected to unrestricted air flow.
The recording graphs were changed weekly. Averages of
daily temperature and daily relative humidity were computed
from the graphs. Also inside the station were two
thermometers. The station was located near the T.W. Bowers
farmstead (Figure 5).
The weather station, test plot equipment and outlying
rain gauges were checked every other day starting June 1,
1981 and continuing through August 1, 1981. The generated
data provides a continuous record of moisture variables for
a large part of the growing season. The following
34
variables were observed for each test plot:
(1) Stemflow, in cubic centimeters, with two
observations or more per plot in five plots,
(2) Percent soil moisture at six, twelve, and eighteen
inches below surface,
(3) Three throughfall measurements,
(4) Crop height in inches, and
(5) Crop cover in percent.
For overall test site conditions, the following variables
were recorded:
(1) Relative humidity,
(2) Temperature,
(3) Three gross precipitation readings.
Throughout the growing season crop conditions such as plant
size, growth, shape, and aging were noted as they related
to stemflow and/or soil moisture.
The 1981 Corn Crop
Corn at the study sites was planted in mid-April 1981.
The field was prepared using normal tillage equipment and
methods, except that only small ridges were formed. By the
time the crop had been cultivated and sprayed there were no
ridges and furrows, the surface was relatively uniform.
After the last sprayings, for weed and insect control, were
completed the Buoyoucos gypsum blocks were implanted and
the test plots established. A hybrid seed corn. Pioneer
3184, was planted in the field where test plots one, two,
and three were eventually located. Dekalb XL34 was planted
three weeks before in the adjoining field where plots four
and five were located. The earliest planted corn had
reached a height for forty inches by June 1, 1981. Though
35
planted later, the field of corn containing plots one, two
and three grew extremely rapid, so that by June 13 both
stands were approximately the same height. Both hybrids
then developed and matured at a similar rate throughout the
remainder of the season. The first tassels appeared by
June 14, and by June 29, ears were forming. The plants in
both fields reached maturity the first week of July. At
this time, plant height and foliage cover had reached their
maximum development. Heights ranged between 8'6" and 9'.
The crop covered at least ninty two percent of the field
surface area with the greatest percentage of cover being
ninty six percent at plots one, two, and three. The crop
was harvested in early September with a yield of 160
bushels per acre for both hybrids, according to Mr. T.W.
Bowers, the owner. Neither hybrid showed signs of stress
at anytime due to a deficiency in moisture.
Precipitation was spread evenly throughout the two
months of record. A total of five rainfall events were of
over one-half inch (12.7 mm.) accumulation. There were
three light rains (0.25 - 2.54 mm.). The crop received a
total of 6.3 inches (160.0 mm.) of precipitation between
June 1 and August 1. In addition, soil moisture content
readings were more than ninety percent for all the test
plots when recording began, indicating a wet Spring month.
Soil moisture was not as uniform as precipitation
throughout the study period, but fluctuated greatly. Each
rainfall event increased soil moisture to one hundred
percent although the gypsum blocks showed approximately a
one day lag in measurement response. Afterwards, soil
moisture decreased in various amounts. Late July was
accompanied by the highest évapotranspiration demands of
the season causing water, available for the plants, to be
almost nil. By then, however, the corn had fully matured
and subsequent crop production was not damaged.
CHAPTER IV
OBSERVATIONS OF PRECIPITATION PATTERNS
In Chapter II a new model of precipitation
distribution was developed. This Idealized Model (Figure
3) depicts a basic precipitation distribution framework.
Into this framework, measurements representing the
variables of stemflow, throughfall, and plant detention can
be added. In this chapter, the Idealized Model (Figure 3)
will be graphically presented to portray each of the
recorded rainfall events of the observation period.
For each rainfall event, gross precipitation and two
of the three idealized model variables, stemflow and
throughfall, were measured. These measurements were
converted to area/depth values. Plant detention was
determined by subtracting stemflow and throughfall from
gross precipitation. Originally, plant detention was not
considered. As the maize stalks cover the ground area,
plant detention tends to be spread evenly over the one
meter section. With the four "water" variable measurements
in their appropriate area and depth amounts, each rainfall
was graphically illustrated using the Idealized Model form
(see Figure 3). The nine rainfalls and the resulting
pattern of water distribution are shown by date of
occurrence. The values used in the illustrations are the
result of a number of samples. These samples were taken
from the five test plots and then averaged. Each variable
38
shown ha d at least 14 sample readings. The drawings of the
rainfall events suggest a pattern of precipitation that
impacted the soil surface for a given maize stalk in the
field. As the illustration s show, each rainfall event was
unique.
There were nine measurable rainfall events recorded
during the period of time from June 1 to August 1 which
comprises the data base, The largest précipitât! on event
in terms of amount of gross rainfall was 50 mm.,
(approximately 2 inches) which occurred on July 6 . There
were five additional rainfalls of over 1 1 mm. ,
(approximately .5 inches) and three rainfalls of re latively
small amounts (0.25 - 2.54 mm. ) .
First Rainfall Event
The first measured rain amounted to 18.0 mm. (.73
inches) (Figure 8). This is an average value derived from
all the rain gauges located in the test area. The maize
had reached between 48 and 54 inches in height, with the
first large wide blades reaching across the rows to touch
the adjacent plants. This type of plant structure produced
a very noticeable funnel shape. The blades on the upper
most section of the stalk were the widest with a
considerable surface area sloping toward the stalk. A
total of 227 mm. of stemflow was collected relative to its
impacting surface area which is a square decimeter. Again,
this is an average value based on a number of sample
^UNE 7’ t 198 1
DUE TO SIZE CONSTRAINTS AREA/DEPTH VALUES
ARE PORTRAYED AT HALF THE LABELED VALUE
figure 8
40
readings. Throughfall quantities were 11 mm., which is
significantly less than the gross precipitation of 18.0
mm. This was postulated to be a result of the interception
by the maize blades of the rain drops that would have
reached the soil surface at a distance of 25mm. - 305 mm.
(1 - 12 inches) from the stalk. Plant detention
constituted only a small portion of gross precipitation.
In this instance, it is assum.ed that the shape and
orientation of the primary blades effected plant
detention. The blades at this time were funneled inward
towards the stalk. A majority of the water drops that
struck the blades did not remain there, but gravitated
toward the ground either to add to throughfall, or in this
case, to stemflow.
Second Rainfall Event
The precipitation distribution pattern of the June 19
rain was very different from the June 7 event. In this
case, 1.71 inches (43.6 mm.) of rain was recorded. This
rainfall event was intense but of short duration, typical
of a summer thunderstorm. While gross precipitation was
significantly greater than June 7, stemflow was much
smaller at 79.9 mm. (Figure 9). Throughfall, on the other
hand, was twice as much at 22.3 mm. Plant detention was
several times that which occurred from the June 7
rainfall. Because the totals of the water distribution
variables from the June 19 event was different from the
JUNE 19, 1981
FIGURE 9
42
earlier rainfall, it can be suggested that rainfall
intensity is a major variable in determining stemflow
efficiency. For example, typical eastern North Carolina
thunderstorms usually begin with large raindrops which
acquire considerable velocity, sufficient to bend or bounce
off the maize blades and pass to the ground. Consequently,
throughfall is increased as stemflow is decreased during
high intensity rainfall events. As the storm wanes the
size and speed of the raindrops decrease. The blades
withstand impact of the latter, redirect this water, and
contribute more to stemflow, but less to throughfall.
During final stages of the storm event, mists or sprinkles
fall. These types of moisture are not sufficient to move
down the blade either as stemflow or throughfall. Also,
these droplets are detained on the blade surface due to the
coarse textured surface of the blade. In this situation,
plant detention comprises a major portion of the available
water from the remaining rainfall. The thunderstorm
activity that produced the June 19 rainfall is postulated
to have contributed to the resulting pattern of fairly even
water distribution among all variables.
Third Rainfall Event
The rainfall of July 2 was very light (Figure 10). A
depth of 0.1 inches (2.5 mm.) of gross precipitation was
recorded. In this case the plant intercepted a majority of
the rain that fell. Only a trace of throughfall was
JULY 2, 1981
FOR CLARITY AREA/DEPTH VALUES ARE PORTRAYED xIO
TO THAT OF THE PREVIOUS DRAWINGS
FIGURE 10
44
measured (0.4 mm.). By this time in the growing season the
maize stalks, and blade were at their peak size and percent
surface cover. The blades touched and in some instances
overlapped between rows. Here is an example where the
plant physiology radically effects the pattern of
precipitation distribution of the soil surface. Stemflow
accounted for 77.1% of all water measured. This stemflow
was directed to the soil surface in the immediate vicinity
of the roots of the stalk where it could potentially be
utilized by the plant. Of the small volume of water that
fell, little was wasted. This indicates that maize,
originating in the Southwest United States as a grass in a
dry climate, has survived through its evolving physiology.
Fourth Rainfall Event
On July 6, two inches (51 mm.) of rain fell, the
largest amount for the season of record (Figure 11). A
period of intermittent showers, at times heavy or light,
produced a typical pattern of variables. Stemflow totaled
129.8 mm., throughfall was 24.1 mm., and plant detention
amounted to 24.0 mm. This precipitation episode saturated
the soil at each test site to a depth of at least 18 inches
beneath the surface. Saturation occurred between the rows
and under the plants in the test beds.
Fifth Rainfall Event
Another rain shower was recorded on July 8 (Figure
12). The shower was at the end of a wet weather period in
JULY 6, 1981
FIGURE 11
JULY 8, 1981
STEMFLOW 13.7mm
FOR CLARITY AREA/DEPTH VALUES ARE PORTRAYED x2
TO THAT OF THE PREVIOUS DRAWING
FIGURE 12
47
the first week of July. The crop was developing the ears
and kernels at this time. Water is needed to "flesh out"
the kernels. Again, stemflow contributed a majority of the
water available to the plant. It is at this critical time
in the growing season that the crop must receive adequate
water. Temperatures are high and sunlight can be intense.
Moisture demands are at their greatest for the season. And
for plant maturation, water must be available for proper
ear formation. Figure 12 illustrates the amelioration
pattern of water potentially available to the plant.
Sixth Rainfall Event
The rain shower of July 16 (Figure 13) fell upon the
maturely developed maize stalks creating a standard or
common picture of water distribution occurence. There was
0.6 inches (15 mm.) of rain recorded. Plant detention was
greater than throughfall (8.5 mm. to 5.8 mm.). The stalks
and blades were fully extended and the stalks laden with
ears. This spread the blades to their maximum extent. In
the fields, the investigator had to crawl under a maze of
blades to reach the test plots as the blades were thickly
intertwined. This being the case, the decreased amount of
throughfall for a given rainfall event is not surprising.
Seventh Rainfall Event
The rainfall distribution for July 18 (Figure 14)
portrays the lowest values that could be recorded with the
equipment used. Even with only a trace of precipitation in
JULY 16, 1981
FIGURE 13
18.1981
STEMflow
FOR CLARITY AREA/DEPTH VALUES ARE PROTRAYED xIO
TO THAT OF THE PREVIOUS DRAWING
figure u
50
the rain gauges, the same pattern of the sixth rainfall
event emerged with stemflow being larger in terms of an
area/depth value, than either throughfall or plant
detention. It is interesting to note that although the
amounts of water described are minimal, in the case of
plant detention, the investigator became thoroughly wet
from the knees up just trying to reach the test plots.
This event was similar to the fifth rainfall event in that
if followed a wet period.
Eighth Rainfall Event
The July 22 event is a good example of stemflow
efficiency on (Figure 15). A fully developed mature maize
crop, just before the onset of the drying stage yielded
stemflow amounts 15 times that of the other moisture
variables. In this case, a half inch of rain occurred in a
steady shower allowing the plant's structure and field
cover to capture incoming water.
Ninth Rainfall Event
The July 30 rain event occurred at a time when the
ears had fully matured (Figure 16). Water demand was
reduced and, in fact, the maize was needing to dry its seed
to prevent rotting. The blades drooped and percent crop
cover was about 60% of the peak coverage. Consequently,
the throughfall increased (10.5 mm.) relative to plant
detention (3.3 mm.). Stemflow remained high (121.1 mm.).
When compared to the previous rainfall event this pattern
15
JULY 30,1981
FIGURE 16
53
shows that a decrease in stemflow is related to an increase
in throughfall which commonly develops as the stalk starts
drying out and the blades droop down. These two rainfalls
had approximately the same amount of gross precipitation,
yet stemflow was greater on July 22 . A total of 0.43
inches (10.9 mm.) fell on July 22 and 0.60 inches (15.2
mm.) on July 30.
S ummary
With the Idealized Model (Figure 3) providing the
framework, measurements of the moisture variables have been
added. The illustrations show each variable in their
appropriate position of impacting the ground surface, and
in their proper amounts. Through the course of the summer
a common pattern emerged, in that stemflow was found to be
of greater volume than both throughfall and gross
precipitation as per the area of impact. A number of
rainfall events had stemflow quantities several times that
of the other variables. Three events showed this large
difference (Figures 8, 15, and 16) . These observed
patterns support the conceptual models in Chapter II.
CHAPTER V
CONCLUSION
Stemflow, as a variable in interception studies has
not been widely studied. There are several reasons for
this, such as time and expense. The difficulty of
measuring stemflow is another. This problem is compounded
when a non-permanent plant, such as maize, is studied.
These problems tend to produce measurement problems related
to stemflow. As a consequence, there is a common
conceptual model of precipitation (see Figure 1), but in
which stemflow is an underestimated and/or hidden
variable. Data gathered from four different years of
record aided in formulating a new conceptual model with the
water variables, appropriately arranged as to the soil
surface and each other (see Figure 2). A final model
simplifies the new model (see Figure 3).
There are four additional concepts concerning
stemflow. These are derived from the 1981 data base and
the three prior years. They deal with stemflow as it
relates to the other parameters in the precipitation soil
system. The concepts ate as follows:
(1) Soil moisture recharge potential changes with respect
to the impact area of stemflow and throughfall in all
directions away from the basal perimeter of the maize
plant (see Equation 4),
(2) Seasonal soil moisture recharge tends to decrease with
distance from the basal perimater of the plant (see
Equation 5),
55
(3) Stemflow efficiency is related to the unit depth
stemflow divided by the unit depth precipitation and
precipitation intensity (see Equation 6), and
(4) Total water available for plant use exceeds both gross
precipitation and throughfall indices (see Equations 7
and 8).
The new model of precipitation distribution (Figure 3)
was formulated to graphically illustrate each rainfall
event. Each event produced a unique pattern of
precipitation. However, a common trend emerged from the
nine illustrations. Stemflow was measured and found to be
a large contributor of water impacting the soil surface
with respect to its areal extent. The results enhanced the
concepts previously stated above.
After most studies are completed, one question
inevitably arises: Where to go from here? In this study a
new conceptual rainfall distribution model was formulated
and stemflow was identified as an important variable of the
model. The next step is to observe and measure a greater
number of stalks for stemflow and throughfall. With an
increase in data, testable hypotheses can be formulated.
The horizontal and vertical movement of water that has
impacted the soil surface would help determine how much the
plant consumes. With a larger number of observations
statistical tests could tell us if what we believe occurs
is true, with some degree of significance. Atmospheric
variables should be recorded as they relate to the rainfall
being measured. In particular, precipitation intensity and
56
its effect on stemflow should be explored. The expense of
such equipment to measure intensity has limited research
thus far. Researchers must constantly observe and monitor
all the field equipment during showers. It is suggested
that frequent checks be made of stemflow and throughfall
collections so that the conceptual models can be tested.
Armed with additional research, models can determine
how a maize crop could best be irrigated. The quantity of
water and precisely where this water should be applied to
the crop will allow better efficiency of the water used.
This is especially important since the human population is
greatly increasing, and with it the need for food, while
the supply of useable water is not.
BIBLIOGRAPHY
Anonymous. 1984. "Water - The Next Resource Shortage?"
Science News, 126 : 39 7
Chang, J.H. 1968. Climate and Agriculture. Chicago:
Aldine Publishing Company.
Ford, E.D., et al. 1978. "Effects of canopy structure on
stemflow, throughfall and interception loss in a young
sitka spruce plantation." Journal of Applied Ecology,
15: 905-17.
Hardy, A. V. 1974. Soil Survey of Pitt County, North
Carolina. Washington, D. C.: U. S. Government Printing
Office. 70
Patrie, J.H. and Helvey, C. 1965. Design Criteria for
Interception Studies. Paper presented at the
International Symposium on Design of
Hydrometeorological Networks, Laval University in
Quebec City, Canada, June 15-22, 1965.
Pressland, A.J. 1973. "Rainfall Partitioning by an Arid
Woodland (acacia aneura F. Muell.) in Southwestern
Queensland" Australian Journal of Botany, 21: 235-45.
Snead, R. 1981. "Irrigation Seminar." Sponsored by North
Carolina State University and conducted at Elizabeth
City, North Carolina (Sept., 1981).
Steila, D. 1976. Unpublished Manuscript.
. 1981. "Field Crop Climatology: An Evaluation
of Stemflow in Maize. Program Abstracts: Association
of American Geographers.
. 1982. Unpublished Manuscript.
. 1983. Conversation. July.
Steila, D. and Wall, E.T. 1977. "A Preliminary Evaluation
of Stemflow in a Crop of Maize." Paper presented
before the North Carolina Academy of Science.
. 1978 . "Stemflow In Its Relation To Soil
Moisture." Paper presented before the North Carolina
Academy of Science.
58
Stephenson, R.A. 1985. "Contingency Analysis of Pitt
County Soils." Paper presented before North Carolina
Academy of Science.
United States Department of Agriculture. 1974. Soil Survey
of Pitt County, North Carolina. Washington, D. C.:
U. S. Government Printing Office.
. 1975. Soil Taxonomy. Washington, D.C.: U.S.
Government Printing Office.