Texas High Plains Research Articles

Ecosystem Evapotranspiration: Challenges in Measurements, Estimates, and Modeling

Abstract. Evapotranspiration (ET) processes at the leaf to landscape scales in multiple land uses have important controls and feedbacks for local, regional, and global climate and water resource systems. Innovative methods, tools, and technologies for improved understanding and quantification of ET and crop water use are critical for adapting more effective management strategies to cope with increasing demand for freshwater resources under global climate change. This article introduces an ASABE Special Collection of 12 articles on ET monitoring and modeling research for multiple land uses and scales. The collection focuses on recent advances in four critical topical areas: (1) reference ET (REF-ET) method development and applications, including crop management and irrigation scheduling, limitations due to sensor inaccuracies and variability, and sensitivity to climatic drivers (three articles); (2) ET process and pathway characterization, including canopy interception, transpiration, and soil evaporation measured using various state-of-the-art techniques on crop lands and plantation forests, and effects of soil moisture on grassland water balance (three articles); (3) ET simulation within hydrological models (SWAT, MIKE SHE, RZWQM, RZ-SHAW, RegCM-BATS, DRAINMOD-FOREST, and Thornthwaite water balance) as well as related processes, such as crop growth and ET/PET ratios, for grass, crop, and forest lands (four articles); and (4) geospatial technology applications, such as using remote sensing to estimate ET and its components (soil evaporation and transpiration) for various land uses (two articles). Study sites represent a range of spatial scales and ecohydrological settings, including grasslands in Inner Mongolia dry lands in northern China, semiarid high plains in Texas, corn production regions from Iowa to Colorado, forest plantations on the humid Atlantic Coastal Plain, developed coastal areas on the island of Taiwan, and the continental U.S. Results from these studies will help guide current development and assessment of REF-ET, ET, and monitoring and modeling of their components in multiple scales and ecosystems. The studies also establish a platform for addressing potential inaccuracies in data from weather sensors and algorithms used in remote sensing products for estimating ET and its parameters, including uncertainties in REF-ET estimates, for tall forest vegetation in particular. Furthermore, the studies offer insights into the interactions between climatic variability and change and vegetation through the ET process.

Calibration and Validation of the SWAT Model for Predicting Daily ET over Irrigated Crops in the Texas High Plains Using Lysimetric Data

<abstract> <b><sc>Abstract.</sc></b> The Soil Water Assessment Tool (SWAT) is a widely used watershed model for simulating stream flow, overland flow, and sediment, pesticide, and bacterial loading in response to management practices. All SWAT processes are directly dependent on accurate representation of hydrology. Evapotranspiration (ET) is commonly the most significant portion of the hydrologic cycle, especially for irrigated lands in semiarid environments when ET demands are met or exceeded. However, no studies using long-term data to evaluate the SWAT model‘s capacity to estimate daily, monthly, and seasonal ET have been performed. In this study, daily and monthly ET values were simulated using ArcSWAT 2012 for an irrigated lysimeter field at the USDA-ARS Conservation and Production Research Laboratory (CPRL) at Bushland, Texas, and compared to measured ET values from 2001-2010. Crops grown during the study period included cotton, soybean, grain and silage sorghum, sunflower, and corn silage. A one-year warm-up (2000) and equal division of the remaining years were used for the calibration (2001-2005) and validation (2006-2010) periods. SWAT achieved a Nash-Sutcliffe efficiency (NSE) of 0.67 for daily ET during the calibration period, resulting in a “good” performance rating. An NSE of 0.78 resulted in a “very good” rating for the validation period. NSE values for simulated average monthly ET were improved, at 0.77 and 0.85 for the calibration and validation periods, respectively. Analysis of simulated versus measured ET values both during and outside of the growing season revealed better agreement during the former than the latter. The SWAT model generally underestimated ET at both daily and monthly levels but overestimated annual cotton ET due to overestimation of leaf area index during the senescing stage. Overall, SWAT was able to simulate daily and average monthly ET reasonably well for major summer crops grown in the semiarid Texas High Plains. These results should reinforce confidence in the SWAT model‘s capacity to accurately simulate ET in fully irrigated watersheds. However, limitations in accuracy appear to exist for certain crops, such as cotton and sunflower, and particularly under limited irrigation conditions. These deficiencies may be related to issues with the embedded Leaf Area Index (LAI) crop growth model and default plant parameter values in the crop database in SWAT.

Spatial Uniformity in Sensitivity Coefficient of Reference ET in the Texas High Plains

<abstract> <b><sc>Abstract. </sc></b>Evapotranspiration (ET) is a major component of the agricultural water budget and accurate ET estimations are essential for effective irrigation scheduling. Therefore, it is an important aspect of production agriculture and agricultural research. Potential crop ET can be calculated by multiplying reference ET by an appropriate crop coefficient. Reference ET can be estimated from meteorological data obtained from weather stations sited on a reference crop surface, e.g., grass or alfalfa. However, weather station sensor inaccuracies may propagate measurement errors into the reference ET calculations. A sensitivity analysis of weather variables used in calculating ET (air temperature, wind speed, solar radiation, and relative humidity) can be used to determine relative effects of data inaccuracies on ET estimates, and conversely, the sensor accuracy levels needed for acceptably accurate ET estimates. In this study, sensitivity analyses of weather variables on daily reference ET were conducted using hourly data (1995-2008) from eight weather stations managed by the Texas High Plains ET network in the Texas Panhandle. Sensitivity coefficients were calculated for each of the weather variables. Comparison of the sensitivity coefficients for any one station indicated that reference ET was most sensitive to wind speed followed by air temperature. Comparison of sensitivity coefficients for any one weather variable (e.g., wind speed) across the region showed no significant differences in the sensitivity of reference ET to those variables by location. This indicates that the existing North-South air temperature gradient and variability in local wind circulation patterns have limited influence on the sensitivity of reference ET to individual weather variables. This indicated that a uniform maintenance protocol should be sufficient for weather stations located throughout the Texas High Plains.

Climate change projections for the Texas High Plains and Rolling Plains

Potential changes in future climate in the Texas Plains region were investigated in the context of agriculture by analyzing three climate model projections under the A2 climate scenario (medium–high emission scenario). Spatially downscaled historic (1971–2000) and future (2041–2070) climate datasets (rainfall and temperature) were downloaded from the North American Regional Climate Change Assessment Program (NARCCAP). Climate variables predicted by three regional climate models (RCMs) namely the Regional Climate Model Version3–Geophysical Fluid Dynamics Laboratory (RCM3-GFDL), Regional Climate Model Version3–Third Generation Coupled Global Climate Model (RCM3-CGCM3), and Canadian Regional Climate Model–Community Climate System Model (CRCM-CCSM) were evaluated in this study. Gaussian and Gamma distribution mapping techniques were employed to remove the bias in temperature and rainfall data, respectively. Both the minimum and maximum temperatures across the study region in the future showed an upward trend, with the temperatures increasing in the range of 1.9 to 2.9 °C and 2.0 to 3.2 °C, respectively. All three climate models predicted a decline in rainfall within a range of 30 to 127 mm in majority of counties across the study region. In addition, they predicted an increase in the intensity of extreme rainfall events in the future. The frost-free season as predicted by the three models showed an increase by 2.6–3.4 weeks across the region, and the number of frost days declined by 17.9 to 30 %. Overall, these projections indicate considerable changes to the climate in the Texas Plains region in the future, and these changes could potentially impact agriculture in this region.

Comparison of infiltration flux in playa lakes in grassland and cropland basins, Southern High Plains of Texas

Playas are the dominant wetland type on the Southern High Plains of Texas and capture runoff during periods of heavy rainfall. Observing the hydrologic functions of playa is important to evaluate their ecological services, which include encouragement of species biodiversity and recharge of the underlying High Plains (Ogallala) Aquifer. Ten pairs of playas were chosen in 10 counties on the Texas Southern High Plains. Each pair included 1 playa surrounded by natural grassland (not in the Natural Resources Conservation Service’s Conservation Reserve Program) and 1 playa surrounded by cultivated cropland. Instrumentation at each playa allowed calculation of changes in free water evaporation and water stored over time during the hydroperiods, defined as continuous durations of surface water storage in the playa basins, caused by one or more rainfall events that generated sufficient runoff flows to reach and fill the playas. A water budget model calculated daily infiltration flux through the playa bottoms. Six cropland playas and 3 grassland playas had significant hydroperiods with associated consistent instrumentation operation during the 6-year study across the years 2005 to 2011. The average observed infiltration flux rates were approximately 10 millimeters/day (range 2 to 20 millimeters/day) and 3 millimeters/day (range 1 to 5 millimeters/day) for the cropland and grassland playas, respectively. The preliminary results may be influenced by the presence of eroded sediments from the surrounding cropland, but more runoff events are needed to differentiate between the impacts of playa floor soils and variations in rainfall and playa watershed characteristics that contribute to the hydroperiods. Citation: Ganesan G, Rainwater K, Gitz D, Hall N, Zartman R, Hudnall W, Smith L. 2016. Comparison of infiltration flux in playa lakes n grassland and cropland basins, Southern High Plains of Texas. Texas Water Journal. 7(1):25-39. Available from: https://doi.org/10.21423/twj.v7i1.7007.

Estimating preseason irrigation losses by characterizing evaporation of effective precipitation under bare soil conditions using large weighing lysimeters

Irrigation from the Ogallala aquifer is used to supplement insufficient precipitation for agricultural crop production in the semi-arid Texas High Plains. Decreased pumping capacity has compelled many producers to “pre-water” fields to field capacity prior to planting to hedge against pumping limitations later in the season. However, the direct measurement of evaporative losses from preseason irrigation of bare soil is not commonly studied. The quantification of evaporative losses from effective precipitation, or the net amount of water that infiltrates into the soil following a precipitation event, can be used as a surrogate for estimating losses from preseason irrigation. We identified 35 precipitation events that occurred over lysimeter fields under fallow conditions in 2002, 2005, and 2009. Events were categorized into four bins of precipitation magnitude ranging from 3mm to 35mm. Subsequent evaporation was measured for a period of up to seven days following rainfall events using large weighing lysimeters at the USDA-ARS Conservation and Production Research Laboratory in Bushland, TX. An exponential decay function was used to characterize bare soil evaporation using maximum cumulative measured evaporation (ECmax), soil water transfer constant (k), and cumulative grass reference evapotranspiration (ETCos). The wide range of ECmax values and k values demonstrated the sensitivity of evaporative losses to both antecedent soil water content and evaporative demand. We also present measured average daily evaporation values for a range of evaporative demand regimes for each precipitation bin. From data analyzed in this study, nearly all of the water from precipitation events of 10mm and less were lost to evaporation within the following day under moderate to high grass reference evapotranspiration (ETos) conditions. Nearly all water from precipitation events between 20 and 30mm was lost to evaporation between three to four days following the event under similar evaporative demand. The considerable potential evaporative losses from preseason irrigation call into the question the prudence of the preseason irrigation, particularly for regions with limited groundwater resources.

El Niño Southern Oscillation Effects on Dryland Crop Production in the Texas High Plains

Risk averse dryland crop management in the U.S. southern High Plains stabilizes annual yields, however in some years the full yield potential is unrealized thereby reducing the overall cropping system productivity. Equatorial Pacific sea surface temperature anomalies (SSTA) systematically couple with the atmosphere to produce predictable El Niño‐Southern Oscillation (ENSO) weather in much of North America that could be exploited for improved crop management. Our objective was to evaluate ENSO effects on site‐specific growth and yield of wheat (Triticum aestivum L.) and grain sorghum [Sorghum bicolor (L.) Moench] in a dryland wheat–sorghum–fallow (WSF) rotation from 1954 to 2011. Measurements from contour farmed terraces were grouped by common ENSO phase years based on 3‐mo classification ending in June (April–June, AMJ) or August (June–August, JJA) that permits timely management of sorghum or wheat. Compared with years classified during September to November (SON) or mature ENSO phases, the JJA classification identified 78 and 68% of common La Niña and El Niño years, respectively, and may have potential application to wheat management. Poor agreement between SON and AMJ phase classifications reduced common La Niña and El Niño years to 61 and 53% respectively, and may limit grain sorghum management. Compared with La Nina, wheat growing‐season precipitation increased 31% for El Nino and contributed to a 55% increase in detrended grain yield differences. For the Texas High Plains, the ENSO phase effects on sorghum grain yield were less pronounced than for wheat, suggesting ENSO based management is better suited to wheat.Core Ideas El Niño phases affected site‐specific rain and crop yield of a 58‐yr field test. Wheat but not sorghum yields responded significantly to El Niño conditions. Predicted El Niño phase has the potential to guide site‐specific wheat management.

Evaluation of Stable Isotopes of Water to Determine Rainwater Infiltration in Soils under Conservation Reserve Program

The Conservation Reserve Program (CRP) is a USDA program introduced in 1985 to reduce soil erosion by increasing vegetative cover of highly erodible land. Participation in the CRP is done via contracts (10 - 15 years in length) and currently the total area of land under contract is set to decline as per the 2014 Farm Bill. The Texas High Plains (THP) leads the US with >900,000 ha enrolled in CRP. A potential long- term benefit of CRP is to increase soil organic matter and to improve soil structure leading to increased water infiltration. Our objective was to evaluate the feasibility of using stable isotopes of water to measure and compare infiltration of rain in land under CRP management to land under continuous dryland cotton in the THP. For this purpose we selected two sites, with soils in the Amarillo series, enrolled in CRP, one for 25 years and the second site for 22 years. Results from several rain events showed that stable isotopes of water are a method that can be used to evaluate the depth of rainwater infiltration for soils under CRP and dryland management.

Urease Inhibitor for Reducing Ammonia Emissions from an Open-Lot Beef Cattle Feedyard in the Texas High Plains

Urease Inhibitor for Reducing Ammonia Emissions from an Open-Lot Beef Cattle Feedyard in the Texas High Plains

Restoring sand shinnery oak prairies with herbicide and grazing in New Mexico

Abstract Sand shinnery oak (Quercus havardii) prairies are increasingly disappearing and increasingly degraded in the Southern High Plains of Texas and New Mexico. Restoring and managing sand shinnery oak prairie can support biodiversity, specific species of conservation concern, and livestock production. We measured vegetation response to four treatment combinations of herbicide (tebuthiuron applied at 0.60 kg/ha) and moderate-intensity grazing (50% removal of annual herbaceous production) over a 10-year period in a sand shinnery oak prairie of eastern New Mexico. We compared the annual vegetation response to the historical climax plant community (HCPC) as outlined by the U.S. Department of Agriculture Ecological Site Description. From 2 to 10 years postapplication, tebuthiuron-treated plots had reduced shrub cover with twice as much forb and grass cover as untreated plots. Tebuthiuron-treated plots, regardless of the presence of grazing, most frequently met HCPC. Tebuthiuron and moderate-intensity grazi...

Ability of Cotton on the Texas High Plains to Compensate for Pre-bloom Square Loss and Impact on Yield and Fiber Quality

In the High Plains region of Texas it is not unusual for cotton (Gossypium hirsutum L.) to suffer pre-bloom fruit loss from insect injury or abiotic factors. The objective of this study was to investigate cotton’s ability to compensate pre-bloom square loss and determine the impact this loss had on yield and fiber quality. Experiments were conducted in Lubbock, TX in 2009, 2010, and 2011 evaluating the impact of 0 to 100% pre-bloom square removal on cotton yield and fiber quality. During 2009 and 2010, cotton was able to either compensate up to 100% pre-bloom fruit loss, or suffered environmentally induced square loss resulting in limited boll carrying capacity that equalized yields and boll density across square removal treatments. If compensation was the reason for the lack of differences among treatments, the plants appear to compensate by producing more secondary or tertiary position fruit, as well as retaining additional mid- to upper-canopy fruit. However, evidence suggested that fiber quality may decline in some compensated bolls due to maturity issues. When environmental conditions were harsh, as in 2011, the cotton plant did not appear to be able to compensate fully for yield due to some pre-bloom fruit loss. This was most evident when an early-season termination event, such as a freeze, was simulated. Where cotton was allowed to mature fully, overcompensation was evident in 2011, with the greatest yields occurring at approximately 27% pre-bloom square removal. As with previous years, fiber quality in 2011 might be impacted adversely from compensated fruit.

Application of DSSAT-CERES-Wheat model to simulate winter wheat response to irrigation management in the Texas High Plains

Winter wheat (Triticum aestivum L.) is a major crop in the semi-arid Texas High Plains. Irrigated winter wheat production in this region mainly depends on water from the Ogallala Aquifer. However declining water levels in this aquifer is a major concern for producers and policy makers. A modeling study was conducted using the DSSAT-CERES-Wheat model for accurate prediction of winter wheat grain and biomass yields and water use efficiency (WUE) responses to irrigation management in the Texas High Plains. Model calibration was performed using field observations of winter wheat response to nine irrigation treatments ranging from dryland to full irrigation. Close match of simulated crop phenology, grain and biomass yields, and evapotranspiration (ET) with observed data indicated accurate prediction of these parameters by the model. Results of simulations using historical weather data for 32 years (1980–2012) showed that a single irrigation of 100mm at jointing or booting had 35% higher grain yield than dryland while 140mm at anthesis or grain filling produced 68% higher grain yield compared to dryland. Simulation of biomass yield showed significant advantage of irrigating 100mm at jointing or booting stage compared to 140mm at anthesis or grain filling. Irrigation of 100mm at jointing and 140mm at anthesis (240mm in total) was found to produce similar grain and biomass yields as full irrigation (400mm). Deficit irrigation at grain filling significantly increased WUE compared to full irrigation. Advantage of deficit irrigation was more pronounced at seasons with below average precipitation. These results show the importance of irrigation timing in winter wheat production under water-limited conditions in the Texas High Plains.

Simulating future climate change impacts on seed cotton yield in the Texas High Plains using the CSM-CROPGRO-Cotton model

The Texas High Plains (THP) region contributes to about 25% of the US cotton production. Dwindling groundwater resources in the underlying Ogallala aquifer, future climate variability and frequent occurrences of droughts are major concerns for cotton production in this region. Assessing the impacts of climate change on cotton production enables development and evaluation of irrigation strategies for efficient utilization of groundwater resources in this region. In this study, the CROPGRO-Cotton module within the Cropping System Model (CSM) that is distributed with the Decision Support System for Agrotechnology Transfer (DSSAT) was evaluated for the THP region using measured data from cotton water use efficiency experiments at Halfway over a period of four years (2010–2013). Simulated seed cotton yield matched closely with observed yield during model calibration (average percent error of 0.1) and validation (average percent error of 6.5). The evaluated model was able to accurately simulate seed cotton yield under various irrigation strategies over the four growing seasons. The evaluated CROPGRO-Cotton model was later used to simulate the seed cotton yield under historic (1971–2000) and future (2041–2070) climate scenarios projected by three climate models. On an average, when compared to historic seed cotton yield, simulated future seed cotton yield across the THP decreased within a range of 4–17% when carbon dioxide (CO2) concentration was assumed to be constant at the current level (380ppm) under three climatic model scenarios. In contrast, when the CO2 concentration was assumed to increase from 493ppm (in year 2041) to 635ppm (in year 2070) according to the Intergovernmental Panel on Climate Change (IPCC) A2 emission scenario, the simulated future average seed cotton yield in the THP region increased within a range of 14–29% as compared to historic average yield. When the irrigation amount was reduced by 40% (from 100% to 60%), the average (2041–2070) seed cotton yield decreased by 37% and 39% under the constant and increasing CO2 concentration scenarios, respectively. These results imply that cotton is sensitive to atmospheric CO2 concentrations, and cotton production in the THP could potentially withstand the effects of future climate variability under moderate increases in CO2 levels if irrigation water availability remains at current levels.

Transpiration Efficiency of Grain Sorghum and Maize under Different Planting Geometries

The Texas High Plains are classified as a semiarid region, but have many characteristics of an arid region. Hence, in this region there is a need for technologies that can sustain crop production under limited water conditions. Planting geometries of grain sorghum (Sorghum bicolor L. Moench) and maize (Zea mays L.) were modified to determine the effect of space between plants on transpiration efficiency and its relation to yield parameters. Three planting geometries―clump (3 plants adjacent to each other), cluster (3 maize or 6 sorghum plants in clusters), and control (plants equally spaced) were studied in a greenhouse. The effect of plant geometry on transpiration efficiency (TE) was not significant, but there was a clear trend that the closer the plants were to each other, the greater the TE. The amount of water were required to produce 1 g biomass was 216, 223, and 231 for plants of equal population growing in clump, cluster, and equally spaced geometries, respectively, indicating a modification in microclimate. The clump treatments appeared to be about 2°C cooler, and the relative humidity was about 3% higher than the control plants during part of the day in sorghum study 1. These findings suggest that under limited water conditions there may be ways to manipulate plant growth by plant geometry. However, TE is only one of several factors that affects crop yield, particularly in grain crops. Thus, a package of complementary technologies must be implemented to enhance biomass production and grain yield in dryland areas.

Water Use and Grain Yield in Drought‐Tolerant Corn in the Texas High Plains

Drought is an important factor limiting corn (Zea mays L.) yields in the Texas High Plains, and adoption of drought‐tolerant (DT) hybrids could be a management tool under water shortage. We conducted a 3‐yr field study to investigate yield, evapotranspiration (ET), and water use efficiency (WUE) in DT hybrids. One conventional (33D49) and 4 DT hybrids (P1151HR, P1324HR, P1498HR, and P1564HR) were grown at three water regimes (I100, I75, and I50, referring to 100, 75, and 50% ET requirement) and three planting densities (PD) (5.9, 7.4, and 8.4 plants m−2). Yield (13.56 Mg ha−1) and ET (719 mm) were the greatest at I100 but WUE (2.1 kg m−3) was the greatest at I75. Although DT hybrids did not always have greater yield and WUE than 33D49 at I100, hybrids P1151HR and P1564HR consistently had greater yield and WUE than 33D49 at I75 and I50. Compared to 33D49, P1151HR and P1564HR had 8.6 to 12.1% and 19.1% greater yield at I75 and I50, respectively. Correspondingly, WUE was 9.8 to 11.7% and 20.0% greater at I75 and I50, respectively. Greater PD resulted in greater yield and WUE at I100 and I75 but PD did not affect yield and WUE at I50. Yield and WUE in greater PD (8.4 plants m−2) were 6.3 to 8.3% greater than those in smaller PD (5.9 plants m−2). The results of this study demonstrated that proper selection of DT hybrids can increase corn yield and WUE under water‐limited conditions.

Evapotranspiration, water productivity and crop coefficients for irrigated sunflower in the U.S. Southern High Plains

Sunflower is a diverse crop grown for oil or confectionary uses in the U.S. Southern High Plains, often under irrigation, but its water use, water productivity (water use efficiency) and crop coefficients for irrigation scheduling are not well known for the Texas High Plains. Crop water use (evapotranspiration or ET) was measured in 2009 and 2011 in two 4.4-ha fields using two precision 9m2 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 about 1.7–1.8m above the ground and 1.5-m apart. The sunflower ET averaged 638mm; seed yields averaged 308gm−2; and the lysimeter crop water productivity averaged 0.49kg (dry seed+hull) m−3. Even in the 2011 record drought season with limited soil water reserves, seed yield and oil content appeared similar to those for the 2009 season with greater precipitation and less irrigation requirement. Also, a month later sowing date in 2011, which might occur following an early cotton crop failure, did not appear to greatly affect ET, crop coefficients, or sunflower seed yields. The basal crop coefficients were 0.15 for the initial period after planting (Kcbini) and 1.22 for the peak water use rate at full cover (Kcbmid) based on the daily ASCE short “grass” reference ET (ETos) and FAO 56 climate adjustment. The Kcbmid based on the ASCE taller, rougher “alfalfa” Reference ET (ETrs) was 0.80. Using a thermal-time base (growing degree day) for the crop coefficient did not greatly improve the representation of the crop coefficient. Comparisons of ASCE reference ET computed using hourly versus daily summary data, and for short and tall reference crops, to each other and to FAO 56 reference ET showed that the relationships between reference ET methods varied significantly from one year to the next. This climate effect means that conversions of crop coefficients from one standard ET formulation to another will not be straightforward.

Castor (Ricinus communis L.) tolerance and weed control with preemergence herbicides

U.S. castor production is limited in part due to only one labeled herbicide (trifluralin) for effective weed control. Field studies were conducted in south Texas and in the Texas High Plains in 2008, 2009, 2012, and 2013 to evaluate castor tolerance and weed efficacy following the application of preemergence (PRE) herbicides. During years of little or poor rainfall, no castor injury was noted following the application of PRE herbicides. However, when rainfall occurred soon after application of PRE herbicides, linuron doses of 1.12kg/ha or greater resulted in stunting of castor plants and reduced stands when compared with the non-treated control. Acetochlor, clomazone, pendimethalin, and trifluralin consistently caused the least injury to castor plants. However, castor bean yield in many instances was not affected by herbicide injury. Diuron at doses of 1.12kg/ha or greater and the combination of diuron plus linuron at 0.56kg/ha or greater effectively controlled (> 80%) smellmelon (Cucumis melo L. var. Dudaim Naud.) and Palmer amaranth (Amaranthus palmeri S. Wats), but provided inconsistent control (0–99%) of Texas millet [Urochloa texana (Buckl.) R.Webster]. Results of these studies indicated that castor tolerance to PRE applications of acetochlor, clomazone, and pendimethalin was excellent, thus providing the greatest opportunity for future use in castor production.

Implications of 3 alternative management policies on groundwater levels in the Texas High Plains

Groundwater supply in the Ogallala Aquifer is diminishing at an unsustainable rate, which is affecting the crop and animal production in the region. The desired future condition adopted by the North Plains Groundwater Conservation District states that at least 40% of the volume of groundwater should remain in the Ogallala Aquifer after 50 years in Dallam, Sherman, Hartley, and Moore counties. The main objective of this study is to evaluate the effects of 3 proposed groundwater management policies on future groundwater levels using a calibrated MODFLOW model. The 3 groundwater management policies considered are permanent conversion of 10% of the total irrigated area to dryland production, temporary conversion of 10% of the total irrigated area to dryland production for the first 15 years, and adoption of advances in biotechnology that allow groundwater use reductions at a rate of 1% per year during the next 50 years. Results indicated that if future average groundwater pumping rates are kept at 2010 withdrawal rates, then 50% of groundwater in the Ogallala Aquifer would remain in 50 years, thus meeting the groundwater district's desired future condition in Dallam, Sherman, Hartley and Moore counties. The most favorable impact on diminishing depletion was obtained with the adoption of advances in biotechnology, which would leave 60% of groundwater remaining in 50 years in the study area. Similar results can be obtained if 1% of irrigated cropland is retired per year. Citation: Hernández JE, Gowda PH, Marek TH, Howell TA, Ha W. 2015. Implications of 3 alternative management policies ongroundwater levels in the Texas High Plains. Texas Water Journal. 6(1):86-102. Available from: https://doi.org/10.21423/twj.v6i1.7005.

Why well yield matters for managing agricultural drought risk

Groundwater-fed irrigation has supported growth in agricultural production around the world by allowing farmers to buffer production against the risks associated with variable and uncertain climatic conditions. However, uncontrolled exploitation has also led to rapid rates of groundwater depletion in many semi-arid and arid regions that threaten farmers' long-term capacity to adapt to future climate change and extreme events. Declining well yields, which control the potential rate and feasibility of groundwater abstraction, are likely to restrict adaptation to drought, but this interaction has largely been neglected in previous research. In this study, we present a set of numerical hydro-economic simulations that assess the joint biophysical and economic effects of climate variability and well yield on irrigated agriculture through a case study in the Texas High Plains region of the United States. Our results demonstrate that reductions in well yield will constrain farmers' ability to use irrigation as an adaptive tool, and may have large negative economic impacts on production. Significantly, economic impacts will be greatest during drought events that are projected to increase in frequency and intensity as a result of climate change. We suggest therefore that management of well yields should be a key consideration when evaluating agricultural drought risk adaptation.

Rainwater use by irrigated cotton measured with stable isotopes of water

The natural rate of recharge for some groundwater systems, such as the Southern Ogallala Aquifer in the central United States, is smaller than the rate of annual withdrawals for irrigation. As aquifer depletion continues, efficient management of the water budget gains importance. Of interest is the use of rainwater by irrigated crops during the course of the growing season, as rain can account for an important amount of the water input in the semi-arid climate of the Texas High Plains. We tested the suitability of stable isotopes of water as a method to determine the source of water in the transpiration of field-grown cotton (Gossypium hirsutum L.) plants from either rain or irrigation-water. We selected this method because irrigation water from the Ogallala Aquifer has a stable isotopic signature that can either be enriched or depleted when compared to the isotopic signature of water from any rain event. Cotton petioles were sampled before two rain events of 33mm, and after every 2h for two days. The water in the cotton petioles was extracted using cryogenic vacuum distillation and was analyzed for its isotopic signature. The results showed a shift of 29% from −7 (‰) to −5 δ18O (‰), which is similar to the isotopic signature of the rainwater (−4.2 δ18O (‰)). These results suggest that it is possible to use water stable isotopes to differentiate between rainwater and irrigation transpired by cotton under field conditions.