87th AMS Annual Meeting

Tuesday, 16 January 2007: 11:15 AM
Analysis of the Influence of Irrigation on Hydroclimate in the California Central Valley using Regional Climate Model Ensemble Simulations
214A (Henry B. Gonzalez Convention Center)
Jiming Jin, Lawrence Berkeley National Laboratory/Univ. of California, Berkeley, CA; and N. L. Miller
Agricultural areas in the California Central Valley have been significantly expanded in the last century. The impact of irrigation on the hydroclimate is not well understood due to the lack of densely distributed soil moisture observations at the irrigated areas. Meanwhile, varying locally by crop type and soil conditions in these areas, irrigation processes result in large uncertainties in soil moisture content. To better understand how irrigation affects California's hydroclimate processes, we generated ten-member ensemble simulations using our version of the Penn State-National Center for Atmospheric Research (NCAR) fifth generation Mesoscale Model (MM5) coupled with the Community Land Model version 3 (CLM3). Each MM5-CLM3 simulation was forced by a fixed soil moisture value that varied between 3% and 30% at 3% intervals over the irrigated areas. These simulations were carried out at a 30-km spatial resolution for the period 1 October 1995 to 30 September 1996. The remotely sensed leaf area indices were incorporated into MM5-CLM3 to realistically describe time-evolving crop processes. The results show that when soil moisture increases during the daytime, both surface evaporation and the soil heat flux will increase, lowering the daily maximum surface temperature. However, during the nighttime for drier soil (e.g. soil moisture below 12%), soil heat transfer dominates the surface energy budget, which supplies more energy to the surface with increasing soil moisture, resulting in the higher daily minimum surface temperature. For wetter soil (e.g. soil moisture above 12%) during the same period, evaporation plays a dominant role, which reduces the daily minimum surface temperature with increasing soil moisture. Additionally, irrigation processes appear to increase low-level atmospheric moisture, slightly deceasing solar irradiance to the surface and amplifying precipitation only when the soil moisture is nearly saturated. These processes need to be further quantified when detailed high-quality soil moisture observations are available. In general, modeling results from this study indicate that irrigation in the California Central Valley has a more significant impact on temperature than on precipitation.

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