Assessing the Impacts of Central Valley Irrigation on Downstream Precipitation in the Irrigation-Enabled Community Earth System

Friday, December 16, 2016 - 17:30
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The impact of irrigation on local surface characteristics is well established; soil moisture levels rise with the addition of irrigation water, which leads to an increase in latent heating at the expense of sensible heating. Associated impacts on precipitation are less certain, particularly in regard to both the location (local or non-local) and the magnitude of the atmospheric response. In a previous study that used the Community Atmosphere Model (CAM3) coupled to the Community Land Model (CLM3.5), Lo and Famiglietti (2013) found that adding irrigation to California’s Central Valley (CV) might increase summer precipitation over the Colorado Basin by up to 15%. In their study, however, irrigation was applied at a fixed rate over the course of the growing season, without dependence on local water resources such as precipitation or soil moisture. Modern versions of CAM5 and CLM4 coupled within the Community Earth System Model (CESM1) now include updated soil hydrology and an option for explicit irrigation parameterization, which applies water only when necessary based on soil moisture and crop leaf area. To advance the understanding of interactions between irrigation and precipitation, we revisit the Lo and Famiglietti (2013) study using newer model versions in CESM1, applying irrigation only in the Central Valley, and conducting a series of sensitivity experiments with varying levels of irrigation. We analyze changes in local evapotranspiration and surface fluxes, as well as the impact of CV irrigation on non-local atmospheric variables. Our results highlight a key difference with the previous study; excess irrigation water in CESM1 leads to a significant enhancement of local runoff, particularly at irrigation levels similar to those used by Lo and Famiglietti (2013). The irrigation parameterization in CLM4 seems to hit an infiltration threshold in the soil column at the default irrigation intensity, shifting the partitioning of excess water from latent heating to runoff. As a result, the non-local precipitation response to irrigation is less pronounced relative to previous studies and difficult to distinguish from internal atmospheric variability.

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