Research > Agrohydrology

Assessing grazing management impacts on hydrology, and soil and water quality at the ranch and watershed scales

Both rural and urban populations depend on ecosystem services provided by rangelands. Ecosystem services can include maintaining stable and productive soils, delivering clean water, and sustaining plants, animals and other organisms that support livelihoods and human aesthetic and cultural values. It is therefore important for ranch/land managers to adopt management practices that maintain or restore soil and ecosystem health and resilience. Grazing management practices have a significant influence on water catchment functions and soil health.

The overall goal of this project is to assess the ranch and watershed scale impacts of traditional continuous grazing and alternate adaptive multi-paddock (AMP) grazing practices on key ecosystem services provided by rangelands, and suggest best grazing management practices using the SWAT and APEX models. The specific objectives are to:

1. Evaluate the impacts of light and heavy continuous, and AMP grazing management practices on water storage, water erosion, water quality, nutrient retention, soil carbon sequestration and downstream flooding risk in selected watersheds in the Southern and Northern Great Plains.
2. Assess the impacts of climate variability and change on water catchment functions, sediment and nutrient losses, and streamflow characteristics under different grazing management practices, and suggest potential climate change adaptation/mitigation strategies.

Study Results

Results from the Clear Creek watershed (71% rangelands) study in north central Texas indicated that the simulated annual surface runoff, and sediment and nutrient losses reduced by about 31%-40% under the AMP grazing when compared to traditional heavy continuous grazing. AMP grazing has also reduced the simulated highest annual streamflow by 25% and hence demonstrated the potential to reduce the risk of flooding downstream.

Results from the Lower Prairie Dog Town Fork Red River Watershed study in Texas Panhandle showed that the AMP grazing was an ideal grazing management practice for streamflow stability, and reduction of flood and drought risk. AMP grazing decreased streamflow by 23.6% and Richards-Baker flashiness Index by 35.9% while increasing Baseflow Index by 31.6% when the compared to the traditional heavy continuous (HC) grazing. When compared to HC grazing, AMP grazing was also found to increase soil organic carbon (SOC), and improve soil ecosystem and hydrological functions. When the grazing management was changed from the baseline HC grazing to AMP grazing, average annual SOC increased by 7.5%. The biomass C and soil nitrogen increased, while C losses from runoff, and sediment loads decreased under AMP grazing.

Development and evaluation of efficient irrigation and crop management strategies for crop production under current and future climatic conditions

Agriculture in the semi-arid Texas High Plans and Rolling Plains region is facing many challenges from rapid declines in groundwater levels, recurring droughts in the recent times, and projected warmer and drier summers in the future. We use the DSSAT Cropping System Model to develop and evaluate environmentally and economically sustainable cropping systems and production practices for the region under the current and future climate change scenarios.

The specific objectives of this study are to:

1. Develop and evaluate efficient irrigation and crop management strategies for crop production, and formulate decision support tools using the evaluated crop modules. Some examples include:
   • Determine the optimum periods for terminating irrigation for cotton in the Texas High Plains.
   • Evaluate efficient crop-growth-stage-based deficit irrigation strategies for cotton and grain sorghum production in the Texas High Plains.
   • Evaluate the feasibility of growing winter wheat cover crop in cotton production systems of the Texas High Plains and Rolling Plains and determine optimum cover crop termination dates.
2. Development and evaluation of a novel sensor- and crop-model based decision support tool for efficient irrigation management
3. Assess potential yield increases through management of soil hydrologic processes in semi-arid dryland agricultural systems
4. Assess the impacts of historic and future climate variability and change on crop production and water use, and suggest climate change mitigation/adaptation strategies.

Study Results

• The optimum irrigation termination dates for 55%, 70%, 85%, 100% and 115% ET-replacement strategies were identified as September 12, September 5, September 5, August 29, and August 29, respectively under normal weather conditions at Halfway in the Texas High Plains. The optimum irrigation termination periods were found to be similar or a week earlier/later than those in normal years in wet/dry years.
• Skipping irrigation during the peak bloom growth stage resulted in the lowest cotton yield, crop water productivity (CWP) and net returns under all weather conditions at Halfway in the Texas High Plains. Skipping irrigation during the seedling emergence/germination stage was identified as the most efficient irrigation strategy for maximizing yield, CWP and net returns.
• Simulated future seed cotton yield showed mixed responses across the Texas High Plains region due to differences in baseline temperature. The irrigated seed cotton yield is expected to increase by 12–21% at cooler northern study sites, and decrease by 2% at the warmer southern study site, in the mid-century compared to the baseline. For the same period, seasonal irrigation water use is expected to increase by 6–11% and dryland seed cotton yield is expected to change by +6% to −11% across the locations. High yielding ideotype was found to be optimum for irrigated conditions with substantial yield gains and marginal increase in irrigation water use. For dryland conditions, the long maturity ideotype simulation resulted in maximum yield gains.
• A prototype of novel sensor- and crop-model based irrigation management decision support mobile app, idCROP (irrigation decision support system for Conserving Resources and Optimizing Crop Production) was developed. This cutting-edge real-time crop irrigation scheduling tool uses historic and short-term forecasted weather data, in conjunction with crop management information provided by the user, to suggest efficient irrigation strategies, so users can choose a strategy that best fits their well capacity and yield/economic goals.

Assessing the impacts of biofuel-induced land use change on watershed hydrology and water quality

The increasing demand for land for biofuel production in the U.S. has led to increased competition for productive agricultural land, shifts in land use among different crops, and conversion of land from other uses into biofuel production. USDA estimates that approximately 11.4% of existing croplands and pastures in southeastern region of the U.S. will be required for second-generation biofuel production. The overall goal of this study is to assess the hydrologic and water quality impacts associated with the change in agricultural land use to biofuels-dominated cropping systems in the semi-arid Southwestern U.S. Cotton Belt region using the SWAT, APEX and Integrated SWAT-APEX models. The specific objectives are to:

1. Assess the impacts of potential land use change from cotton to cellulosic bioenergy crops such as Alamo switchgrass, Miscanthus, big bluestem and biomass sorghum on water balances and water quality at the landscape and watershed scales.
2. Study the effects of historic and future climate variability on water balances, sediment and nutrient loads, and crop yields under baseline and biofuel-induced land use change scenarios.

Study Results

Results from the Double Mountain Fork Brazos watershed in the Southern High Plains of Texas indicated that Miscanthus and switchgrass would serve as ideal bioenergy crops for the dryland and irrigated systems, respectively. This is due to their higher water use efficiency, better water conservation and water quality improvement effects, greater biomass and biofuel production potential, and minimum crop management requirements.

Replacing cotton with perennial grasses (switchgrass in irrigated areas and Miscanthus in drylands) decreased simulated annual surface runoff, total nitrogen load through surface runoff and nitrate leaching to groundwater by 88%, 86% and 100%, respectively and increased percolation by 28%. The climate change analysis indicated that the simulated annual irrigation water use and total nitrogen load under the future perennial grass land uses would reduce by 60% and 30%, respectively, when compared to future cotton land use.

Assessment of spatio-temporal variability of groundwater quality and availability in Texas

Texas is largely dependent upon groundwater resources. About 59% of state’s total water supply and about 99% of the rural household needs are met from the groundwater extracted from 9 major and 21 minor aquifers of the state. Future projections, however, indicate about 30% reduction in water availability over the next few decades due to adverse climatic conditions and depletion of major aquifers. Numerous studies have documented groundwater quality degradation in several parts of the state, which threatens community welfare and sustainable development.

In the face of snowballing crises of water availability and water quality deterioration, the overall objective of this study is to identify long-term spatio-temporal trends in groundwater levels and groundwater quality across the state and unravel the nexus between the two.

We integrate different geochemical, graphical, and statistical techniques within a geospatial environment to seek answers to questions such as:

1. Where are the hotspots of groundwater contamination and groundwater level declines?
2. Is groundwater contamination a manifestation of changes in groundwater levels?
3. What are the potential causes of groundwater contamination and groundwater level declines?
4. What are the effects of different agricultural and land management practices on groundwater quality and availability?
5. What are the best management practices for conserving groundwater resources and protecting water quality?

Study Results

Our long-term assessment of groundwater quality (nitrates, fluoride and salinity) in several major and minor aquifers of Texas has provided more insights into various factors that affected groundwater quality and led to identification of groundwater quality degradation hotspots. We have also delineated spatially associated zones of groundwater level declines in Texas, and identified hotspots that warrant implementation of appropriate management strategies. The state-wide decadal median groundwater levels in Texas were found to decline from about 14 m from land surface in the 1930s to about 36 m in the 2000s. Groundwater level declines across the state, however, mostly followed logarithmic trends marked by levelling-off phenomena in recent times due to implementation of conservation measures and regulatory strategies.

Quantification of improvements in ecosystem services from adoption of soil health promoting practices

Maintenance of healthy soil ecosystems is a key for ensuring water, food and energy security for current and future generations. The health of soils is determined by the ecosystem services they provide on the farm/ranch, across the watershed and downstream of the watershed, which include harvestable phytomass, soil carbon sequestration, rainfall infiltration and retention, soil fertility, plant nutrient acquisition and nutrient cycling, and disease resistance. Practices such as conservation/no-tillage and cover crops reduce soil erosion, add organic matter to soil, increase infiltration and soil water holding capacity, increase nutrient retention, reduce leaching, break pest cycles, provide resources for beneficial insects, and thereby improve soil health.

The overall goal of this project is to simulate field- and watershed-scale improvements to ecosystem services due to long-term adoption of soil health promoting practices in the lower and middle Brazos River Basin in Texas using the PALMS and APEX models.

Study Results

1. Results from a field-scale study at Riesel in the Texas Blackland Prairies region indicated that the no-tillage (NT) management improved plant-available water over conventional tillage (CT) during the corn growing season, especially in dry years. On an average, NT captured about 23 and 9 cm more rainfall in dry and normal years, respectively when compared to CT.
2. Results from the watershed-scale in the Brushy Creek Watershed in the Texas Blackland Prairies region showed that changing tillage management from CT to NT on all croplands (35% of the watershed area) resulted in a 25% (38 mm/year) reduction in average annual runoff and a 57% (1.13 t/ha/year) reduction in average annual sediment loss at the outlet of the watershed.

   

   Measurement of soil water from cover crop experimental fields at Chillicothe, TX

   
   

Enhancing soil ecosystem health and resilience through pasture cropping

Pasture cropping is a farmer-initiated, relatively new, and innovative land management system that integrates direct seeding of annual crops into dormant perennial grasses. Although the U.S. Southern Great Plains region provides an excellent platform for this regenerative approach that originated in Australia, studies evaluating soil health benefits of pasture cropping are lacking.

The overarching goal of this study is to evaluate improvements in soil-dependent ecosystem services when grasslands are pasture cropped. We have initiated pasture cropping field experiments at the Pittman Ranch near Muenster, TX and the Nance Ranch near Canyon, TX. We will evaluate soil ecosystem service benefits of pasture cropping at the ranch- and watershed-scales using the APEX model and we will analyze short- and long-term profitability implications of pasture cropping in comparison to conventional practices. Our study will provide first quantitative measures in the US regarding the effects of pasture cropping on soil health and profitability of perennial grasslands and changes in on- and off-site ecosystem services. Other potential outcome is enhanced rancher awareness about the soil health and climate change mitigation effects of pasture cropping.