Brian A. Joyce

INFILTRATION AND SOIL WATER STORAGE UNDER WINTER COVER CROPPING IN CALIFORNIA’S SACRAMENTO VALLEY

Winter cover cropping on agricultural fields may improve rainfall infiltration and enhance soil water storage in areas such as California’s Sacramento Valley, where the majority of precipitation occurs in the winter over a relatively short period of time in a series of heavy rainfall events. Enhanced soil water storage within the root zone on cover–cropped fields may benefit a grower by reducing the demand for surface water deliveries to meet the irrigation needs of subsequent crops. A study was conducted in the winters of 1998–1999 and 1999–2000 to determine a field’s ability to conserve water for subsequent crops and to evaluate the effects of soil physical conditions on the water balance for three 4–year rotation farming systems within the Sustainable Agriculture Farming Systems (SAFS) Project at the University of California, Davis. Rainfall, runoff, and soil water content data was collected on two treatments using a winter cover crop and one treatment maintained fallow during the winter. Runoff and soil water content measurements were significantly affected by farming systems. Cumulative event runoff from 10.67 m 2 infiltration test areas was consistently higher on the fallow treatment than on the cover–cropped treatments. Winter 1999–2000 field water content measurements from 0–1.05 m depth were significantly higher in the cover–cropped systems than in the fallow treatment after field capacity had been reached. A hydrologic model was developed using the measured data and lysimeter data for evaporation and evapotranspiration to track daily water budget components (i.e., runoff, infiltration, evaporation, evapotranspiration, and soil water storage) and to assess changes in surface hydraulic conductivity. Model simulations showed that optimized hydraulic conductivity decreased for all treatments with successive runoff, but was less pronounced in cover–cropped plots. The study indicated that cover cropping can improve soil water storage for subsequent crops if the cover crop is destroyed before the additional soil water is lost as evapotranspiration.

Potential Impacts of Climate Warming on Water Supply Reliability in the Tuolumne and Merced River Basins, California

We present an integrated hydrology/water operations simulation model of the Tuolumne and Merced River Basins, California, using the Water Evaluation and Planning (WEAP) platform. The model represents hydrology as well as water operations, which together influence water supplied for agricultural, urban, and environmental uses. The model is developed for impacts assessment using scenarios for climate change and other drivers of water system behavior. In this paper, we describe the model structure, its representation of historical streamflow, agricultural and urban water demands, and water operations. We describe projected impacts of climate change on hydrology and water supply to the major irrigation districts in the area, using uniform 2°C, 4°C, and 6°C increases applied to climate inputs from the calibration period. Consistent with other studies, we find that the timing of hydrology shifts earlier in the water year in response to temperature warming (5–21 days). The integrated agricultural model responds with increased water demands 2°C (1.4–2.0%), 4°C (2.8–3.9%), and 6°C (4.2–5.8%). In this sensitivity analysis, the combination of altered hydrology and increased demands results in decreased reliability of surface water supplied for agricultural purposes, with modeled quantity-based reliability metrics decreasing from a range of 0.84–0.90 under historical conditions to 0.75–0.79 under 6°C warming scenario.

Modeling the Transport of Spray-Applied Pesticides from Fields with Vegetative Cover

This study presents the development and qualitative evaluation of numerical and analytical solutions for the transport of spray-applied pesticides in runoff. The model development focuses on the enhancement of the solution algorithms of earlier researchers by adding chemical reactions that represent more completely the pesticide exchange processes between vegetation, soil, and overland flow and including reactions that are consistent with the short time spans encountered during experimental rainfall/runoff events. The influence of these revised assumptions was evaluated by comparing model outputs from the different models. These tests revealed that for bare soil cases model simulations diverge only where rates of desorption from soils become limiting. Differences in simulation results were more dramatic where a vegetative cover was present, due to divergence in assumptions on the movement of water between plant surfaces and overland flow. Parameter testing revealed several key model parameters that influence simulation results: depth of mixing zone, soil-water partition coefficient, and kinetic rate of desorption from soil solids. A comparison of numerical and analytical model formulations demonstrated that a closed-form solution was adequate to capture the general response of pesticides in runoff that was estimated with a more detailed numerical solution.

Analysing Stakeholder Driven Scenarios with a Transboundary Water Planning Tool for IWRM in the Jordan River Basin

Although IWRM has become the mainstream concept for water management, its implementation in transboundary, politically tense settings, such as the Jordan River basin, is still limited. In this study we present the application of a transboundary spatially explicit water resources simulation and planning tool in support of decision making in this contentious setting. We integrated socio-economic scenarios and water management strategies resulting from a stakeholder process, thereby including socio-economic uncertainty, using the WEAP modelling software. Tool development was supported by an active transboundary dialogue between scientists and stakeholders. The tool was used to identify water scarcity effects and spatial-temporal response patterns under four regional scenarios up to the year 2050. These scenarios suggested that the positive effects of large scale water management options such as sea water desalination and the increased use of treated wastewater can be strongly limited by insufficient water transport infrastructure and/or a lack of cooperation. Respective responses to water scarcity should be pursued with the same intensity as currently the implementation of large scale supply-side options.