Soil Science Society of America Journal 64:311-318 (2000)
© 2000 Soil Science Society of America
DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION
Soil Water Components Based on Capacitance Probes in a Sandy Soil
A. Faresa and
A.K. Alvab
a University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Rd., Lake Alfred, FL 33850 USA
b USDA-ARS-PWA, 24106 N. Bunn Rd., Prosser, WA 99350 USA
aalva{at}tricity.wsu.edu
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ABSTRACT
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Understanding soil water movement is needed to manage irrigation to minimize water drainage, nutrient leaching below the root zone, and contamination of groundwater. We hypothesized that soil water content determined by capacitance probes can be used for irrigation scheduling and estimating soil water components. Objectives of this study were (i) to evaluate the performance of capacitance probes for optimizing irrigation management for `Hamlin' orange trees [Citrus sinensis (L.) Osb.] on Swingle citrumelo [Citrus paradisi Macf. x Poncirus trifoliata (L.) Raf.] rootstock on a Candler fine sand soil (hyperthermic, uncoated, Typic Quartzipsamment) in Central Florida and (ii) to determine soil water balance components. Irrigation levels were determined based on available soil water (ASW) and tree growth stage. The soil water data measured at finite time interval by capacitance probes were used with irrigation and rainfall data to calculate daily evapotranspiration (ET) and drainage rates. Daily ET rates showed strong seasonal patterns and varied from <0.4 mm d-1 in January to 5 mm d-1 in July and August. The annual ET in 1997 was 920 mm or 53% of the total water input (irrigation and rainfall). The cumulative annual drainage in 1997 was 890 mm, or 47% of the total water input. Furthermore, 82% of the cumulative annual drainage was contributed by rainfall. Irrigation based on monitoring soil water content using capacitance probes minimized water drainage below the root zone in a system where rainfall contributed substantially to drainage.
Abbreviations: ASW, available soil water • BMP, best management practices • ET, evapotranspiration • ET0, daily potential evapotranspiration
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INTRODUCTION
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MAINTENANCE OF ADEQUATE soil water content through most of the crop growing period is necessary to support optimum plant growth and yields. In most growing regions, soil water is at optimum level only for a short portion of the growing season; hence, irrigation is needed to maintain adequate soil water availability to support optimal production and quality. The purpose of well-managed irrigation is to optimize water spatial and temporal distribution, to promote crop growth and yield, and to enhance citrus economic returns. Hence, the aim is not necessarily to obtain the highest yields per unit area of land, or per unit volume of water, but to maximize net returns.
When water is applied on the soil surface, assuming little or no surface runoff, a portion of water is utilized by the plants, or retained in the soil, and the excess water drains through the vadose zone into the groundwater, which contributes to aquifer recharge. This excess water may contain agricultural chemicals and soluble nutrients. Irrigation best management practices (BMP) are designed to (i) minimize water and nutrient leaching below the root zone to maintain adequate irrigation water within the rooting depth, (ii) minimize non-point source pollution of groundwater, and (iii) reduce production costs associated with water and nutrient losses by leaching.
Supply of water to crops must be based on a clear understanding of the soil water dynamics. The water cycle of an agricultural field comprises evapotranspiration, irrigation, rainfall, runoff, and drainage losses below the root zone. Under-tree sprinklers and drip irrigation systems are designed to deliver water at rates low enough so that the soils can retain the water without contributing to losses by runoff or excessive downward drainage through the soil.
Using these low-volume irrigation systems, it is possible to schedule small but frequent irrigation events to maintain an optimum water content in the root zone while decreasing water losses below this depth. Under such conditions, it is possible to minimize the water drainage below the root zone. This is in marked contrast with high drainage rates following low-frequency and high-volume irrigation events (Hillel, 1990).
Given the close relationship between excess irrigation and water losses, an understanding of the dynamic components of the soil water balance is important to manage irrigation properly. A water budget requires the accounting of all water input and output components, including rainfall, irrigation, drainage below the root zone, runoff, evapotranspiration, and changes in soil water storage. One-dimensional analytical models (Alva et al., 1999; Darusman et al., 1997; Chopart and Vauclin, 1990) and one dimensional numerical models (Lotse et al., 1992; Jabro et al., 1995) were used to estimate field water drainage fluxes below the root zone.
The objectives of this study were (i) to evaluate the use of capacitance probes to optimize citrus irrigation and (ii) to use collected data to estimate the components of soil water balance under young citrus tree growing conditions.
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Materials and methods
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The experiment was conducted in a research farm of the University of Florida Citrus Research and Education Center at Lake Alfred (280°01' N., 81°055' W), Polk county, Florida, using a 4-yr-old citrus grove of Hamlin orange trees on Swingle citrumelo rootstock planted (at 7.6 by 4.6 m spacing) in a Candler fine sand in September, 1993. This soil is a well-drained sandy soil with no restricting soil layer and, therefore, a deep water table that will not influence the hydrology of this system. The trees were irrigated with under-tree low-volume sprinklers (80% efficiency) using one emitter per tree, with a delivery rate of 50 L h-1 covering an area of 7.3 m2, which resulted in an irrigation rate of 7 mm h-1 (
20% of the total grove area). The soil water release curves for the different soil horizons were presented by Alva et al. (1999). The mean bulk density values were 1.59, 1.52, 1.51, 1.61, and 1.55 g cm-3, and saturated hydraulic conductivities were 5.21, 9.48, 8.52, 7.27, and 6.96 m d-1, respectively, for the soil horizons at 0- to 20-, 20- to 50-, 50- to 100-, 100- to 130-, and 130- to 200-cm depths (Alva et al., 1999). The rainfall events were measured using two rain gages placed in two separate locations on the grove; however, irrigation rates were determined based on timing and flow meter readings. The rainfall and irrigation events during 1997 are shown in Fig. 1
. The annual rainfall for 1997 was 1470 mm, which was 22% greater than the long-term mean annual rainfall of 1200 mm for this location. A recommended fertilizer program (Tucker et al., 1995) was followed. In addition, a standard herbicide program (Knapp, 1994) was followed to maintain a 210-cm-wide herbicide band on both sides of the trees. The herbicide band was maintained weed-free throughout the year.
A Real-Time Soil Water Monitoring System
The soil water content through the soil profile was continuously monitored using three EnviroSCAN capacitance probes (EnviroSCAN, Sentek PTY Ltd., South Australia).1
Sensors for each of the probes were installed within the emitter wetting area under canopy along the tree drip line of randomly selected trees at 10-, 20-, 40-, 70-, and 110-cm depths. These probes were connected to a solar-powered data logger, and soil water readings were recorded at 10-min intervals. The first three depths represent the depth of rooting (A.K. Alva, 1999, unpublished data), while the last two depths represent the soil profile below the rooting depth. A schematic of the location of a probe is shown in Fig. 2 . Each probe was installed under the tree drip line 50 to 60 cm away from the low-volume sprinkler to assure a good irrigation coverage of the sensors.
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Fig. 2 Schematic of the location of sprinkler and the EnviroSCAN capacitance probe under the tree canopy
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EnviroSCAN systems have been extensively tested under laboratory (Mead et al., 1995; Paltineanu and Starr, 1997) and field conditions (Buss, 1993; Starr and Paltineanu, 1998). They have been used for soil water content monitoring and irrigation scheduling of different crops in the USA (Fares et al., 1998; Starr and Paltineanu, 1998) and in Australia (Buss, 1993). Soil water content data were recorded on a data logger located up to 500 m away from the access tube (Fig. 3)
.
The capacitance method measures the apparent dielectric constant of the soil surrounding the sensor, which reflects the water content of the soil–water–air mixture. The dielectric constant of a medium depends on the polarization of its molecules in an electric field. Because the dielectric constant of water (80) is large compared with that of the soil matrix (<10) or air (1), small changes in soil water content strongly influence the dielectric constant of the soil–water–air mixture. This relationship between water content change and dielectric constant of the medium depends on soil type and frequency range of the measuring apparatus. Because of their high frequency range (150 Mhz), measurements by EnviroSCAN sensors are relatively insensitive to changes in fertilizer salts over typical concentrations used in agricultural cropping systems (Gardner et al., 1991; Paltineanu and Starr, 1997). Paltineanu and Starr (1997) studied the response of the capacitance sensors subjected to a wide range of air and water temperatures. They found that errors due to temperature variations between 10 to 30°C were less than the root mean square error for their calibration curve. Measured frequencies are converted to water content using the calibration curve supplied by the manufacturer. These sensors are capable of measuring volumetric water content values ranging from a saturated soil to almost oven-dry soil with a resolution of 0.1% (Buss, 1993).
Irrigation Scheduling
EnviroSCAN water content readings through the soil profile were downloaded every other day to schedule irrigation or process data. The target refill points for optimal irrigation scheduling were based on an evaluation of the allowable soil water depletion within the rooting depth of 40 cm. The ASW was determined as:
 | (1) |
where 
FC and PWP are the volumetric water content (cm3 cm-3) at field capacity and at the permanent wilting point (equivalent to soil matric potential of -1.5 MPa), respectively. The field capacity term was determined based on the work of Fares and Alva (2000). They found that Candler fine sand soil holds between 0.08 to 0.10 (cm3 cm-3) of water 1 d after saturation, with little or no surface evaporation. Thus, the volumetric water content at field capacity was taken as 0.09 (cm3 cm-3). The permanent wilting point (0.015 cm3 cm-3) was selected based on laboratory measurements (Sodek et al., 1990). Therefore, the ASW is 0.075 (cm3 cm-3). The ASW per unit area for the top 40 cm, which is the effective rooting depth, is 0.075 by 40 cm, or 3 cm.
Optimal citrus production requires maintaining soil water content above the 33% depletion of the ASW during the period from February to May to avoid potential adverse effects of water stress on flowering and fruit set (Koo, 1969). However, during the remaining part of the growing season, ASW can be allowed to deplete by 67% before replenishment of the soil water back to field capacity. Depletion of 33 and 67% of the ASW to a soil depth of 40 cm corresponds with equivalent water content of 2.6 and 1.6 cm, respectively. The goal of each irrigation event was to deliver an adequate amount of water to replenish the deficit in the top 40 cm to field capacity. This target point is defined as the full point, which is equivalent to 3.6 cm
for the target depth of irrigation (40 cm). Thus, irrigations were applied when the total water content in the root zone reached 2.6 and 1.6 cm during the first period from February to May and from June to January, respectively.
Data Processing Using a Utility Program
A utility program, SENTPROC, was developed which converts the water content data stored in the data logger into spreadsheet format. SENTPROC was coded using the FORTRAN programming language (a copy of the executable file of SENTPROC can be obtained from the authors). Using a spreadsheet and knowing the depth of the root zone, the user can calculate the soil water content for the entire soil depth of probe installation and also for within and below the root zone depths (Fig. 4)
. These data are useful for determining irrigation scheduling and for calculating the different water balance components (i.e., evapotranspiration and drainage below the root zone). Data downloaded from the data logger were processed using SENTPROC.
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Fig. 4 A sample data showing (A and B) soil water content at each depth and (C) depth-integrated water content within the root zone (0–40 cm) and below the root zone (40–110 cm) of 4-yr-old Hamlin orange trees on a Swingle citrumelo rootstock in a Candler fine sand
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Calculation of Drainage and Evapotranspiration
The field water balance of a citrus grove grown on a sandy soil can be defined based on the conservation of mass as follows:
 | (2) |
where, 
S is the change in water storage in the root zone, R is the rainfall, I is the irrigation, D is the drainage at the bottom of the root zone, and ET is the evapotranspiration during a given period of time t. All of these variables are expressed as millimeters. Runoff component was not considered in Eq. [2] since most of Florida sandy soils have high hydraulic conductivities and very little slope, resulting in negligible runoff. Irrigation and rainfall were directly measured by rain gages and flow meters, respectively. Water storage within and below the root zone was calculated using the soil water measurements by the capacitance probes (Eq. [6]). The remaining unknown terms are D and ET (Fig. 5)
.
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Fig. 5 Flow chart explaining the steps taken during the calculation of ET, drainage, and storage variations through the soil profile
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Drainage below the root zone was estimated using Darcy's equation since the water content at the bottom and below the rooting zone are known at short time intervals. The water flux across the plane at depth z can be computed using Darcy's flow equation:
 | (3) |
where, k() (cm d-1) is the effective unsaturated hydraulic conductivity at the water content (cm3 cm-3) of the soil layer below the rooting zone; t is the time for which the drainage was computed (1 h was used); h (cm) is the pressure head gradient between the bottom of the rooting zone depth and next depth in the profile where the water content is monitored; z (30 cm) is the distance between the bottom of the rooting zone (40 cm) and the next depth in the soil profile where the water content is known (70 cm).
van Genuchten (1980) derived an analytical expression that relates the unsaturated hydraulic conductivity to the water content. The relationship between the unsaturated hydraulic conductivity and the water content is:
 | (4) |
where,
, s (cm3 cm-3) is the water content at saturation; r (cm3 cm-3) is the residual water content; (cm3 cm-3) is the water content at which k is being calculated; m is a fitting parameter; and ks (cm d-1) is the saturated hydraulic conductivity.
Knowing the water content at a given location of the soil profile, the pressure head at that same location can be estimated by the following equation (van Genuchten, 1980):
 | (5) |
where, 
(cm-1) and n are fitting parameters.
The variation in soil water storage (S) between two depths (
and
) for a given period of time (
; i.e., 1 h was used) was calculated based on measured water content readings by the capacitance probes using the following equation:
 | (6) |
Variation in water content below the root zone is due to water redistribution into this depth from the soil profile above it. Thus, any net increase in the water content in the soil horizon immediately below the root zone depth can be used as a first approximation of water losses below the root zone of the soil profile.
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Results and discussion
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Soil Water Storage
Volumetric soil water contents measured using the capacitance probes were used to calculate the water content for the entire profile. Figure 6
shows the mean water content (of three capacitance probes randomly placed in the field) for the 0- to 40-, 40- to 110-cm, and 0- to 110-cm depth profile. Irrigation was scheduled based on the recommended refill points depending on the tree growth stages. Accordingly, the soil water content in the root zone was maintained within 33% depletion of the ASW during the critical period from February to June and at 67% depletion of the ASW during the rest of the year. From Fig. 6 it is evident that the water content in the root zone was maintained within the target refill points during the entire growing season, except for few occasions during February to June when the water content in the root zone dropped slightly below the refill point just before the subsequent irrigation event. The sharp increase and decrease in the water content following each irrigation or rain event clearly demonstrates the low water-holding capacity and high hydraulic conductivity of this soil. Fares and Alva (2000) reported that water content in this Candler fine sand soil decreased to 0.09 cm3 cm-3 within a day after saturation as a result of a large irrigation or rainfall.
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Fig. 6 Depth-integrated soil water content (A) within and (B) below the root zone and for (C) the entire monitored soil profile depth during 1997. The refill points indicate the soil water content at which irrigation was scheduled to replenish the water deficit. The allowable depletion of the available soil water (ASW) was 33% in February to June, and 67% during the rest of the year
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The increase in water content in the soil immediately below the root zone is due to water drainage from the root zone. During the first 90 d of the year, the water content below the root zone varied around 40 mm, except on 19 January when it increased to 60 mm as a result of 42 mm of irrigation applied as a precautionary measure to protect against forecasted freezing temperatures. In April, the water content below the root zone showed large increases in response to heavy rainfall events. This was due to a net total rainfall of 174 mm, which was nearly fivefold greater than the long-term average. May and June were drier than the average. This explains the decrease in the water content below the root zone as compared with its level during April. The remaining portion of the year was in the rainy season, and an unusually wet fall resulted in excess water drained below the root zone.
Drainage
The daily drainage varied considerably through the study period (Fig. 7)
. The mean drainage values were as high as 47 mm following a 58-mm rainfall event on 28 May. In general, the daily drainage rates were low during January to March, which represents the dry months when the optimal irrigation is critical for the trees. The low drainage below the root zone during this dry period demonstrates that irrigation scheduling based on near-continuous monitoring of the water content within the root zone minimized drainage. All of the irrigation events were managed to add only enough water to replenish the soil to the full point. The rainfall in April 1997 was fivefold greater than the long-term average and accordingly resulted in several large drainage events as shown in Fig. 7. Greater than 40% of the annual cumulative drainage occurred during October to December (Fig. 7), which were unusually wet months.
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Fig. 7 Calculated daily mean, standard deviation, and cumulative mean values of water drainage below the root zone of 4-yr-old Hamlin orange trees on Swingle citrumelo rootstock grown in a Candler fine sand during 1997
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In 1997, the annual cumulative drainage below the root zone was 890 mm (Fig. 7) from the 1470 mm of rainfall plus 340 mm irrigation. The 890 value is very close to the upper limit of the range (i.e., 625 and 860 mm) reported by Rogers and Bartholic (1976) for a citrus grove in Central Florida. They calculated the annual water balance based on daily weather data and water storage in three different soils using irrigation input that is 50% lower than that used in our work.
Daily Evapotranspiration
The daily ET values calculated based on Eq. [2] for each calender date for 1997 are shown in Fig. 8
. The seasonal variations in daily evapotranspiration show an order of magnitude difference during the growing season (i.e., <0.4 mm d-1 on some days in January to almost 5 mm d-1 on some days in June through August). Weekly citrus ET estimated by Rogers and Bartholic (1976) using a modified Blaney–Criddle procedure followed a similar seasonal pattern. Their data were for an average mature citrus grove and under an average yearly rainfall (1335 mm) and irrigation (304 mm). Their average daily ET values based on a weekly estimation were as low as 0.3 mm d-1 during the winter months (December–February) and as high as 4.2 mm d-1 during the summer months (June–August).
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Fig. 8 Calculated daily mean, standard deviation, and cumulative values of evapotranspiration for 4-yr-old Hamlin orange trees on Swingle citrumelo rootstock grown in a Candler fine sand during 1997
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Daily evapotranspiration based on total monthly values from a citrus grove with grass understory cover in poorly drained soils in the East coast of Florida ranged between 1.9 and 5.0 mm d-1 (Rogers et al., 1983). These daily averages based on monthly totals mask the large variability of daily ET in every month as is shown in our work (Fig. 8) and in the data by Rogers and Bartholic (1976) and Sumner (1996).
Daily pan evaporation data measured at a weather station in the vicinity of this field experiment site were used with a crop coefficient 0.55 to estimate daily potential evapotranspiration 
. The above crop coefficient value was adapted from data reported by Rogers et al. (1983). Their work showed that the crop coefficient of young citrus trees increased with tree growth, from 0.51 for 1-yr-old trees to 0.65 for 8-yr-old trees. In our study the trees were 4 yr old. The data in Fig. 9 show a strong linear relationship
between and ET with some degree of variability
below and above the 1:1 regression line. This may be due, in part, to spatial variability in the soil water content, which consequently could influence the calculated ET and drainage data. Considering soil variability, some variation in soil properties is expected even within an area as small as 1 m2 of a relatively homogeneous soil profile (Gardner, 1986). Taylor (1955) reported coefficients of variation of 17 and 20% for gravimetric soil water content of samples from soil from field plots under furrow and sprinkler irrigation methods, respectively, for a relatively uniform Millville (coarse-silty, carbonatic, mesic Typic Haploxeroll) loam soil.
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Fig. 9 Linear regression analysis between calculated daily evapotranspiration and potential evapotranspiration for 4-yr-old Hamlin orange trees on Swingle citrumelo rootstock grown in a Candler fine sand during 1997. SEC is standard error coefficient
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Yearly Cumulative Evapotranspiration
The cumulative ET in 1997 for 4-yr-old citrus trees was 920 mm (Fig. 8). The annual ET value from this study is within the range of values reported for citrus in a deep sandy, well-drained soil in central Florida (Koo and Harrison, 1965; Smajstrla et al., 1986) or poorly drained soils with a perched water table in the East coast of Florida (Rogers and Bartholic, 1976; Rogers et al., 1983), in Texas (Wiegand and Swanson, 1982), and in Arizona (Erie et al., 1965). The ET value in this study is 140 mm lower than that reported by Rogers et al. (1983), 200 mm lower than that of Koo (1963), and 520 mm lower than that of Hoffman et al. (1982) for mature citrus in Arizona. This high ET data for citrus in Arizona compared with Florida may be due to the greater evaporative demand (potential evapotranspiration) in Arizona compared with that of Florida. The greater ET reported by Koo (1963) compared with our study can be attributed primarily to the difference in tree age (i.e., 4-yr-old trees in our study vs. mature trees in Koo, 1963) and variation in climatic conditions during different years. Rogers et al. (1983) reported a linear relationship between citrus tree age and annual ET and demonstrated a 13% increase in ET for the 8-yr duration of their study.
Estimated annual ET for a deforested area in the Lake Wales Ridge, FL (Sumner, 1996) reached 680 mm. The low ET values reported by Sumner (1996) compared with the ET for commercial citrus in this study can be attributed to the lack of irrigation and shallow-rooted natural vegetation. Irrigation in commercial citrus groves produces a wet soil, and more water will be available for plant uptake, which causes a higher actual ET than the nonirrigated native vegetation. Hoffman et al. (1982) reported a 48% greater annual citrus ET for a frequently irrigated grove compared with that for a less frequently irrigated grove used by Erie et al. (1965). Actual ET is strongly related to evaporative demand (potential ET) and ASW for plant uptake. Consequently, a given crop species with a given potential ET has a greater ET in a wet soil than in a dry soil.
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Summary and conclusions
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Results of this experiment demonstrated that monitoring of soil water using capacitance probes can be used to optimize irrigation scheduling for citrus groves on a sandy soil. Given the knowledge of soil water characteristic curves, effective rooting depth, and recommended depletion of ASW content depending on the crop growth stages, the root zone soil water can be replenished to its optimum level while minimizing drainage and avoiding plant stress. The data provided by the capacitance probes can be used to determine the components of citrus water balance. The water balance method was used to determine the daily evapotranspiration. Cumulative annual evapotranspiration and drainage were 920 and 890 mm, respectively. Most of the drainage occurred during the summer months and the unusually wet fall. Daily evapotranspiration varied seasonally, ranging from 0.4 mm d-1 in January to 5.0 mm d-1 in July and August. These daily ET values were well within the daily ET values reported for citrus trees in Florida.
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ACKNOWLEDGMENTS
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This study was made possible by partial funding from the Florida Citrus Production Research Advisory Council.
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NOTES
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Contribution of the Citrus Research and Education Center. Florida Agricultural Experiment Station Journal Series no. R-06632.
1 Brand name and distributor are given to provide specific information and do not constitute endorsement of the product by the authors or by the University of Florida 
Received for publication October 1, 1998.
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TOP
ABSTRACT
INTRODUCTION
