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Title: EFFECT OF THE ACCURACY OF SPATIAL RAINFALL INFORMATION ON THE MODELING OF WATER, SEDIMENT, AND NO3-N LOADS AT THE WATERSHED LEVEL

Author
item CHAPLOT, V - IRD-L'INST. DE RECHERCHE
item SALEH, A - TARLENTON STATE UNIV.
item Jaynes, Dan

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2005
Publication Date: 10/11/2005
Citation: Chaplot, V., Saleh, A., Jaynes, D.B. 2005. No-till corn response to banded K on Des Moines lobe soils. Journal of Hydrology. 312(1-4):223-234.

Interpretive Summary: Chemicals that leach through soil pose contamination threats to surface and groundwater quality and can ultimately impact human health. It is difficult and expensive to monitor chemical movement and fate below the surface of soils. Computer models can be used instead of direct measurements to determine chemical fate and movement, but the soil properties needed to run the models are also difficult to measure. In this research, we developed and tested a new technique for measuring the soil properties that are needed for modeling water and chemical movement in soil. The simple method uses irrigation drip emitters and a chloride tracer and can give multiple soil measurements in a limited amount of time. This technique and the soil parameters it measures will be of use to scientists and engineers interested in determining the movement of water and chemicals in soils. Ultimately, the technique developed here will be of use to decision makers and others for regulating chemical use in the environment.

Technical Abstract: The accuracy of watershed model predictions of the hydrological response to rain events and the resulting sediment and nitrate loads within the stream depends on the quality of the precipitation data used to drive the model. Because rainfall can be highly variable both across the landscape and over time, little is known as to how many locations within a watershed rainfall needs to be measured in order to derive accurate model predictions. In this paper, we use rainfall and stream data collected from two watersheds in widely different climates -- the Bosque River watershed in central TX and the Walnut Creek watershed in central IA -- to determine the optimum number of rainfall locations needed for accurate predictions. Within each watershed data from 1 to 15 rain gages were used to simulate stream flow and sediment and nitrate loads over a 9-year period using the ARS Soil and Water Assessment Tool (SWAT). We showed that the full compliment of rain gages was required to obtain accurate predictions of stream flow and sediment load in both watersheds. We also showed that using national weather station data from locations within 100 km of the watersheds rather than on-site measurements produced inaccurate predictions. Finally, we generalized the results from this study to other watersheds and models. This research will benefit a wide range of modelers, researchers, and action agency personnel conducting watershed research and monitoring activities.