Steven R. Evett

The Bowen ratio-energy balance method for estimating latent heat flux of irrigated alfalfa evaluated in a semi-arid, advective environment

The Bowen ratio-energy balance (BREB) is a micrometeorological method often used to estimate latent heat flux because of its simplicity, robustness, and cost. Estimates of latent heat flux have compared favorably with other methods in several studies, but other studies have been less certain, especially when there was sensible heat advection. We compared the latent heat flux of irrigated alfalfa (Medicago sativa, L.) estimated by the BREB method with that measured by lysimeters over a growing season in the semi-arid, advective environment of the southern High Plains. Difference statistics from the comparison and indicators of sensible heat advection were used to analyze the performance of the BREB method relative to lysimeters. Latent heat flux was calculated from mass change measured by two precision weighing lysimeters and from two BREB systems that used interchanging temperature and humidity sensors. Net radiation (Rn), soil heat flux ( G), and other meteorological variables were also measured. Difference statistics included the root mean square difference (RMSD) and relative RMSD (normalized by mean lysimeter latent heat flux). Differences between lysimeters averaged 5‐15% during the day, and 25‐45% at night. Estimates of latent heat flux by the two BREB systems agreed closely (relative RMSD D8%) when they were at the same location with sensors at the same height. Differences increased when the location was the same but sensors were at different heights, or when the sensor height was the same but location in the field different, and probably was related to limited fetch and the influence of different source areas beyond the field. Relative RMSD between lysimeter and BREB latent heat fluxes averaged by cutting was 25‐29% during the first two cuttings and decreased to 16‐19% during the last three cuttings. Relative RMSD between the methods varied from 17 to 28% during morning hours with no pattern based on cutting. Afternoon relative RMSD was 25% during the first two cuttings and decreased to 15% during subsequent cuttings. Greatest differences between the two methods were measured when the Bowen ratios were less than 0, on days that were hot, dry and windy, or when the latent heat flux exceeded the available energy ( Rn G). These conditions were likely to be encountered throughout the growing season, but were more common earlier in the season.

Soil hydraulic properties of cropland compared with reestablished and native grassland

Conversion of cropland to perennial grasses will, over time, produce changes in soil hydraulic properties. The objective of this study was to characterize and compare hydraulic properties of fine-textured soils on adjacent native grassland, recently tilled cropland, and reestablished grassland in the Conservation Reserve Program (CRP) at three locations in the Southern Great Plains. A tension infiltrometer was used to measure unconfined, unsaturated infiltration over a range of supply pressure heads (nominally, h=−150, −100, −50, and −5 mm H2O) at the soil surface. Intact soil cores were sampled within the Ap and Bt horizons to determine bulk density and water desorption curves, θ(h), at potentials ranging from −0.15 to −100 kPa. Unsaturated hydraulic conductivity K(h) over the range in supply pressure heads was estimated using Woodings equation for steady-state flow from a disc source. The van Genuchten water retention model was fitted to θ(h) data to estimate parameter values. Soils in CRP had greater surface bulk densities than their grassland and cropland counterparts. The shape of the soil water retention curve for grassland and CRP land were similar, suggesting that converted croplands had fully reconsolidated. Mean near-saturated hydraulic conductivities of cropland at h=−5 mm were not significantly different from grassland. However, at −150 mm supply pressure head, cropped soils had a mean unsaturated conductivity 2.3 and 4.1 times greater than CRP land and grassland, respectively. Sites in CRP had the lowest (P<0.05) near-saturated hydraulic conductivities (h=−5 mm), which suggest that after 10 years, grasses had not fully ameliorated changes in pore structure caused by tillage. Comparison of unsaturated conductivities for grassland and CRP land suggest that long-term structural development on native grasslands was principally confined to effective pore radii greater than 300 μm. Land use practices had a greater effect on water movement than did soil series, indicating that the modifying effects of tillage, reconsolidation, and pore structure evolution on hydraulic properties are important processes governing water movement in these fine-textured soils.

SUBSURFACE AND SURFACE MICROIRRIGATION OF CORN —SOUTHERN HIGH PLAINS

Microirrigation has the potential to minimize application losses to evaporation, field runoff, and deep percolation; improve irrigation control with smaller, frequent applications; supply nutrients to the crop as needed; and improve crop yields. This study was conducted to evaluate subsurface and surface microirrigation (SUB and TOP, respectively) application methods on crop performance. The effects of irrigation frequency, amount, and application method on crop yield, yield components, water use, and water use efficiency of corn (Zea mays L., cv. PIO 3245) were investigated at Bushland, Texas, on a slowly permeable soil [Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll)] in a semi-arid environment in 1993 and 1994. Irrigation frequencies were once a day and once a week; irrigation levels varied from dryland (no post emergence irrigation) to full crop water use replenishment; and application methods were on the soil surface (TOP) and 0.3 m below the surface (SUB) with emitters spaced 0.45 m apart and drip lines spaced 1.5 m apart. Irrigation frequency and application method did not affect crop yields; however, deficit irrigation affected crop yields by reducing the seed mass and the seed number. On the clay loam soil at Bushland, irrigation frequency and application method are less critical than proper irrigation management for microirrigation systems to avoid water deficits that have a larger affect on corn yields.

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.

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.

COMPARISON OF SDI, LEPA, AND SPRAY IRRIGATION PERFORMANCE FOR GRAIN SORGHUM

Subsurface drip irrigation (SDI), low-energy precision application (LEPA), and spray irrigation can be very efficient by minimizing water losses, but relative performance may vary for different irrigation system capacities, soils, crops, and climates. A three-year study was conducted at Bushland, Texas, in the Southern High Plains to compare SDI, LEPA, and spray irrigation for grain sorghum on a slowly permeable Pullman clay loam soil. Performance measures were grain yield, seed mass, soil water depletion, seasonal water use, water use efficiency (WUE), and irrigation water use efficiency (IWUE). Each irrigation method was compared at five irrigation levels: 0%, 25%, 50%, 75%, and 100% of crop evapotranspiration. The irrigation levels simulated varying well capacities typically found in the region and dryland conditions. In all three years, SDI had greater yield, WUE, and IWUE than other irrigation methods at the 50% irrigation level and especially at the 25% level, whereas spray outperformed SDI and LEPA at the 75% and 100% levels. Differences in seed mass, soil water depletion, and seasonal water use were usually insignificant at the 25% and 50% levels and inconsistent at the 75% and 100% levels. Performance was most sensitive to irrigation level, then year, and then irrigation method, although relative rankings of performance for each irrigation method within an irrigation level were consistent across years. For this climate and soil, SDI offers the greatest potential yield, WUE, and IWUE for grain sorghum when irrigation capacities are very low.

Evapotranspiration of Irrigated Winter Wheat — Southern High Plains

Models of water use for irrigation scheduling and for crop growth simulation require validation of the evapotranspiration (ET) submodel. In this study ET was measured for irrigated winter wheat (Triticum aestivum L.) at Bushland, Texas, in the semi-arid Southern High Plains for the 1989-1990, 1991-1992, and 1992-1993 winter wheat cropping seasons using weighing lysimeters that contained undisturbed monoliths 3 ¥ 3 ¥ 2.3 m deep of Pullman clay loam soil (Torrertic Paleustolls). Weather data from a nearby station were used to compute daily ET values for a reference alfalfa crop (hypothetical) using the ASCE Manual No. 70 equations based on the Penman-Monteith equation and several other widely used “potential” or “maximum” ET models. Linear regressions between ET estimated from widely used equations and the reference alfalfa ET equation indicated that direct comparisons with computed ET values could not be reliably predicted with simple ratios. For the computed reference alfalfa ET base, peak basal crop coefficients (Kcb) varied from 0.88 to 1.00 for the three seasons and were lower than those reported from other locations. Peak mean crop coefficients (Kc) varied from 0.83 to 0.94 for the three seasons. Seasonal ET varied from 791 to 957 mm for the three seasons. Evapotranspiration and crop coefficients for winter wheat varied considerably with season.

Evapotranspiration of Corn and Forage Sorghum for Silage

In the U.S. Southern High Plains, dairies have expanded and have increased the regional demand for forage and silage. The objectives were to measure water use and determine crop coefficients for corn (Zea mays L.) and forage sorghum (Sorghum bicolor (L.) Moench) produced for silage on the Southern High Plains. Water use was measured with large, precision weighing lysimeters in 2006 and 2007. Both growing seasons had normal to above normal rainfall. The 2006 season was more advective with greater mean daily reference evapotranspiration (ET) rates. Seasonal ET was 671 mm for forage sorghum with a yield of 1.48 kg m -2 in 2006 and 489 mm in 2007 with a yield of 1.70 kg m -2 ; water productivity was 2.21 kg m -3 in 2006 and 3.47 kg m -3 in 2007. Seasonal ET was 418 mm for corn for silage with a yield of 1.52 kg m -2 in 2006 and 671 mm in 2007 with a yield of 2.44 kg m -2 ; water productivity was 3.63 kg m -3 in 2006 and 3.64 kg m -3 in 2007. Using the 2007 season as a better species comparison, forage sorghum can achieve comparable water productivity as corn with less ET (~73% of corn ET) and irrigation requirement although with a reduced yield (~62% of corn dry matter).