Judy A. Tolk

Remote Sensing Based Energy Balance Algorithms for Mapping ET: Current Status and Future Challenges

Evapotranspiration (ET) is an essential component of the water balance and a major consumptive use of irrigation water and precipitation on cropland. Remote sensing based agrometeorological models are presently most suited for estimating crop water use at both field and regional scales. Numerous ET models have been developed in the last three decades to make use of visible, near-infrared (NIR), shortwave infrared (SWIR), and most importantly, thermal data acquired by sensors on airborne and satellite platforms. In this article, a literature review is done to evaluate numerous remote sensing based algorithms for their ability to accurately estimate regional ET. The remote sensing based models generally have the potential to accurately estimate regional ET; however, there are numerous opportunities to further improve them. The spatial and temporal resolution of currently available remote sensing data from the existing set of earth-observing satellite platforms are not sufficient enough to be used in the estimation of spatially distributed ET for on-farm irrigation scheduling purposes, especially at the field scale (~10 to 200 ha). This will be constrained further if the thermal sensors on future Landsat satellites are abandoned. Research opportunities exist to improve the spatial and temporal resolution of ET by developing algorithms to increase the spatial resolution of surface temperature data derived from ASTER/MODIS thermal images using same/other-sensor high-resolution visible, NIR, and SWIR images.

SEASONAL AND MAXIMUM DAILY EVAPOTRANSPIRATION OF IRRIGATED WINTER WHEAT, SORGHUM, AND CORN — SOUTHERN HIGH PLAINS

Evapotranspiration (ET) is basic information required for irrigation scheduling and for crop growth simulation models. However, many ET models have not been tested for their applicability to the Southern High Plains. In this study, ET was measured for irrigated winter wheat (Triticum aestivum L.), sorghum [Sorghum bicolor (L.) Moench], and corn (Zea mays L.) at Bushland, Texas, in the semi-arid Southern High Plains for various growing seasons from 1988 through 1993. Weighing lysimeters containing Pullman clay loam (Torrertic Paleustolls) monoliths were used to measure ET. Weather data from a nearby station were used to compute daily ET values for several widely used reference or potential ET equations. These computed values were then compared by linear regression with the measured ET values for periods of full groundcover (LAI=3) and with adequate soil water to permit maximum ET. Measured mean seasonal ET was 877 mm for winter wheat, 771 mm for corn, and 578 mm for sorghum. Maximum daily ET rates rarely exceeded 10 mm d–1 for the sorghum or corn crops, except for a few days during a brief period of strong advection in 1990 when corn ET rates exceeded 12 mm d–1. Maximum daily ET for wheat exceeded 10 mm d–1 on many days during the three seasons due to the high vapor pressure deficits and wind speeds at Bushland during the spring and early summer. The Penman-Monteith equation performed consistently better than other combination and/or radiation/temperature based ET equations in estimating maximum daily ET rates for these crops. The leaf diffusion resistance (rl ) permitting the best agreement between predicted and lysimetrically determined ET was 280 s m–1 for sorghum, 252 s m–1 for corn, and 135 s m–1 for wheat when using the relationship of rc = rl /(0.5 LAI) where LAI is the leaf area index and rc is canopy resistance in s m–1. These results indicate that the greater seasonal water use by irrigated corn compared with sorghum in this environment was due mainly to the differences in planting date and growing season length since the “apparent” leaf resistances were similar. The even higher seasonal and maximum daily water use of irrigated winter wheat compared with corn and sorghum was due to its longer growing season, its lower leaf resistance, and the high evaporative demand in the spring in the Southern High Plains.

Role of transpiration suppression by evaporation of intercepted water in improving irrigation efficiency

Sprinkler irrigation efficiency declines when applied water intercepted by the crop foliage, or gross interception (Igross), as well as airborne droplets and ponded water at the soil surface evaporate before use by the crop. However, evaporation of applied water can also supply some of the atmospheric demands usually met by plant transpiration. Any suppression of crop transpiration from the irrigated area as compared to a non-irrigated area can be subtracted from Igross irrigation application losses for a reduced, or net, interception (Inet) loss. This study was conducted to determine the extent in which transpiration suppression due to microclimatic modification resulting from evaporation of plant-intercepted water and/or of applied water can reduce total sprinkler irrigation application losses of impact sprinkler and low energy precision application (LEPA) irrigation systems. Fully irrigated corn (Zea Mays L.) was grown on 0.75 m wide east-west rows in 1990 at Bushland, TX in two contiguous 5-ha fields, each containing a weighing lysimeter and micrometeorological instrumentation. Transpiration (Tr) was measured using heat balance sap flow gauges. During and following an impact sprinkler irrigation, within-canopy vapor pressure deficit and canopy temperature declined sharply due to canopyintercepted water and microclimatic modification from evaporation. For an average day time impact irrigation application of 21 mm, estimated average Igross loss was 10.7%, but the resulting suppression of measured Tr by 50% or more during the irrigation reduced Igross loss by 3.9%. On days of high solar radiation, continued transpiration suppression following the irrigation reduced Igross loss an additional 1.2%. Further 4–6% reductions in Igross losses were predicted when aerodynamic and canopy resistances were considered. Irrigation water applied only at the soil surface by LEPA irrigation had little effect on the microclimate within the canopy and consequently on Tr or ET, or irrigation application efficiency.

COMPARISON OF FIVE MODELS TO SCALE DAILY EVAPOTRANSPIRATION FROM ONE-TIME-OF-DAY MEASUREMENTS

Calculation of regional, spatially distributed evapotranspiration (ET) is possible using remotely sensed surface temperatures from sensors aboard air or space platforms. These platforms provide instantaneous data at frequencies of days to weeks, so that instantaneous latent heat flux can be computed from energy balance algorithms. However, instantaneous latent heat flux must be converted to ET and then scaled to daily (24 h) totals for most practical applications. We compared five scaling models where a single measurement of 0.5 h ET was used to estimate the daily total during clear days. Each model takes advantage of the clear day, quasi-sinusoidal nature of daytime ET and other daytime parameters including solar radiation, available energy, or reference ET. The surfaces were fully irrigated alfalfa, partially irrigated cotton, dryland grain sorghum, and bare soil (tilled fallow sorghum). Actual ET was measured by precision weighing lysimeters. Model agreement was evaluated on the basis the modified index of agreement (D) and the modified coefficient of efficiency (e), in addition to standard statistical parameters. For cropped surfaces, the models based on grass reference ET resulted in the best agreement between observed and predicted daily ET totals. For bare soil, the model based on available energy (i.e., evaporative fraction) resulted in the best agreement. Relative error between observed and predicted daily ET increased as daily ET decreased. Observed and predicted daily ET agreed well for the transpiring crops (RMSE of 0.33 to 0.46 mm d-1 for mean daily ET of 3.9 to 5.8 mm d-1) but poorly for bare soil (RMSE of 0.47 mm d-1 for mean daily ET of 1.4 mm d-1).

A Depth Control Stand for Improved Accuracy with the Neutron Probe

The neutron thermalization method for soil water content measurement is well established as being accurate for deep soil profile measurements. However, the method has been criticized as inaccurate for shallow measurements ( 0.98 and RMSE values of calibration <0.01 m3 m−3. The stand is also useful for elevating the gauge high enough above the surface so that standard counts are not influenced by the water content or nature of the surface, thus enhancing accuracy of both the calibration and subsequent water content readings, both of which depend on standard count values. Also, the stand serves to prevent repetitive strain injuries to backs and knees caused by bending and kneeling to place the gauge on top of access tubes, but without additional occupational exposure to radiation.

Water use efficiencies of grain sorghum grown in three USA southern Great Plains soils

Abstract The ratios of economic yield:evapotranspiration (Y:ET), or water use efficiency (WUE), and economic yield:irrigation water application, or irrigation WUE (IWUE), help evaluate the productivity of irrigation in agricultural systems. Water stress at critical growth stages, excessive soil water evaporation, soil water storage, runoff, and drainage are among the many factors which result in declines in either or both of these ratios. The objective of this research was to evaluate the effect of soil type, soil water use characteristics, and seasonal climatic differences on the WUE and IWUE of grain sorghum grown in the semi-arid climate of the southern Great Plains of the USA. In 1998 and 1999, grain sorghum (Sorghum bicolor (L.) Moench ‘PIO-8699’) was grown in 0.75-m rows with 16 plants/m2 at Bushland, TX in lysimeters containing monolithic soil cores of either the Amarillo, Pullman, or Ulysses soil series. Irrigation treatments in both years were 100, 50, 25, and 0% based on replacement of ET, simulating deficit irrigation that results from limited water availability such as reduced well capacities. The WUE was significantly higher and ET lower in the milder climatic conditions of 1999 compared with 1998, which had a higher evaporative demand. Once normalized for climatic differences, yield response to ET was similar for both years. Crops grown in the Amarillo soil had significantly higher WUE compared with crops in the other soils, primarily due to reduced ET rather than increased yield. Grain sorghum grown in the Ulysses soil was able to produce higher yields at lower plant available water (PAW) compared with the other two soils, but the crops in all soils reduced yield when experiencing water stress at a critical growth stage of pollination. At comparable final soil water contents, grain yields of the crop in the Pullman soil were higher in 1999 (lower evaporative demand) compared with yields produced in 1998 (higher evaporative demand), while the crops in the other two soils produced similar yields in both environments. The relationship between irrigation application and yield was more curvilinear in 1998 possibly due to increased soil water evaporation at the higher irrigation applications, while the relationship was more linear in 1999. In general, IWUE declined with increasing irrigation application within each year, but was variable in some irrigation treatments, due to water stress at critical growth stages. No differences among soil types occurred in IWUE in either year, primarily due to variability among replicates.

Comparison of aerodynamic and radiometric surface temperature using precision weighing lysimeters

Radiometric surface temperature (Ts) is commonly used as a surrogate for aerodynamic temperature (To) in computing the sensible heat flux term (H) in the energy balance. However, these temperatures may differ by several degrees, leading to possible errors (especially for large H) and their relationship is not well known. Previous researchers have established empirical and semi-empirical parameterizations of the radiometric roughness length (zor) or some related form (e.g., kBr−1 = ln[zom/zor], where zom is the momentum roughness length). In this paper, we estimated To - Ta (where Ta is air temperature at 2 m height) and zor using large, precision weighing lysimeters planted with irrigated alfalfa, irrigated and dryland cotton, and dryland grain sorghum. Ts was measured by infrared thermometers mounted over the lysimeters. No apparent relations were found between (To − Ta) and (Ts - Ta) or between zor (in the kBr−1 form) and meteorological variables or leaf area index (LAI). The kBr−1 parameter appeared to be most influenced by the different surface roughness of each crop type. Using constant kBr-1 values established for each type of surface, the energy balance model showed reasonable agreement with H and LE derived from lysimeter measurements.

Grain sorghum growth, water use, and yield in contrasting soils

Abstract Soil characteristics and the climate in which they occur help control crop growth and yield. We conducted a study to determine the influence of contrasting soils on grain sorghum ( Sorghum bicolor Moench) growth, water use, and yield. In 1992 and 1993, grain sorghum (‘DK-46’) was grown in 0.75-m rows with 16 plants m −2 at Bushland, TX in lysimeters containing monolithic soil cores of silty clay loam, silt loam, and fine sandy loam. The 1992 irrigation treatments were well-watered (WW) and no applied early season irrigation to achieve a pre-anthesis water stress. The 1993 irrigation treatments were WW with limited irrigation during late vegetative and reproductive growth stages to achieve a post-anthesis water stress. The crop in the silt loam soil produced lower grain yield in 1993 under high soil water conditions, but greater grain yield, total biomass, and seed number under reduced irrigation compared with the crop on the clay loam. The crop in the sandy loam consistently produced the lowest leaf areas and yield components in all irrigation treatments, possibly due to high soil bulk densities which may have restricted rooting. The 1993 crop in the silt loam had the highest water use in all treatments, and extracted water uniformly throughout the profile in both years. High strength silty clay and clay horizons and possibly a calcic horizon in the silty clay loam may have delayed or limited rooting, and affected crop growth and yield. The crop in the sandy loam consistently produced the lowest yield components in all irrigation treatments, possibly due to restricted rooting resulting from high bulk densities and also low water holding capacity.

Two-Source Energy Balance Model: Refinements and Lysimeter Tests in the Southern High Plains

A thermal two-source energy balance model (TSEB-N95) was evaluated for calculating daily evapotranspiration (ET) of corn, cotton, grain sorghum, and wheat in a semiarid, advective environment. Crop ET was measured with large, monolithic weighing lysimeters. The TSEB-N95 model solved the energy budget of soil and vegetation using a series resistance network, and one-time-of-day latent heat flux calculations were scaled to daily ET using the ASCE Standardized Reference ET equation for a short crop. The TSEB-N95 model included several refinements, including a geometric method to account for the nonrandom spatial distribution of vegetation for row crops with partial canopy cover, where crop rows were modeled as elliptical hedgerows. This geometric approach was compared to the more commonly used, semi-empirical clumping index approach. Both approaches resulted in similar ET calculations, but the elliptical hedgerow approach performed slightly better. Using the clumping index, root mean squared error, mean absolute error, and mean bias error were 1.0 (22%), 0.79 (17%), and 0.093 (2.0%) mm d-1, respectively, between measured and calculated daily ET for all crops, where percentages were of the measured mean ET (4.62 mm d-1). Using the elliptical hedgerow, root mean squared error, mean absolute error, and mean bias error were 0.86 (19%), 0.69 (15%), and 0.17 (3.6%) mm d-1, respectively, between measured and calculated daily ET for all crops. The refinements to TSEB-N95 will improve the accuracy of remote sensing-based ET maps, which is imperative for water resource management.

Evapotranspiration and Crop Coefficients for Irrigated Sunflower in the Southern High Plains

Sunflower (Helianthus annuus L.) is a diverse crop grown for oil or confectionary uses in the Southern High Plains often under irrigation. Crop water use (evapotranspiration or ET) was measured in 2009 and 2011 in two 4-ha fields using two precision 9 m2 weighing lysimeters containing 2.3-m deep monoliths of Pullman clay loam soil. The fields were irrigated with a lateral move sprinkler system with nozzles ~1.7-1.8 m above the ground and ~1.5-m apart. The sunflower ET averaged 638 mm; seed yields averaged 308 g m-2; and the lysimeter water productivity averaged 0.49 kg m-3. The crop coefficients based on the FAO-56 curve method were 0.15 for Kcbini and 1.22 for Kcbmid based on the daily ASCE Reference ET (ETos). The Kcbmid based on the ASCE taller, rougher Reference ET (ETrs) was 0.80. Using a thermal-time base (growing degree day) for the crop coefficient did not improve the crop coefficient for the diverse planting dates in these seasons.