Regional scale simulations of nitrate movement through the vadose zone using Hydrus 1D
LAWR, UC Davis
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Nitrate is recognized as one of California’s most widespread groundwater contaminants since it is frequently detected in groundwater systems. Nitrate enters the soils mainly due to excessive application of nitrogen fertilization for agricultural purposes. The fate of nitrate in the soils, along with the risk of reaching the groundwater is a function of very complex processes at various time and spatial scales. In this study we classify agricultural soils in California by their nitrate leaching risk. Risk is here defined through transient flow and nitrate transport simulations for almost 6000 unique soil profiles using the Richards equation. The parameterization of the model was based on the SoilWeb Database, taking into account heterogeneity of the soil hydraulic properties (SHPs) in the z-axis. Overall, over 22,000 soil horizons were used. In a first step, we calculated nitrate transport through the shallow vadose zone for a simple scenario: Nitrate was modelled as a conservative tracer for a constant rainfall/irrigation scenario. In a second step, we included the effect of climate conditions on nitrate transport. The final goal of this study is to evaluate and include key management, physical, and chemical processes that influence nitrate leaching.Preliminary results show, that SHPs influence strongly the movement of nitrate through soils. We present maps for all the agricultural soils in California, helping identify regions in California which may need improved fertilization management. Furthermore, this analysis allows us to estimate drainage rates of water under the root zone at the regional scale, a very important parameter for assessing the risk of groundwater contamination.