Remote Sensing Crop ET Analysis and Modeling

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Owner: Kaweah Delta Water Conservation District
Location:
 Tulare and Kings Counties, California
Funding Source: Kaweah Delta Water Conservation District
Completion Date: Ongoing

Corn field and irrigation ditch near KDWCD

The Kaweah Delta Water Conservation District (KDWCD) is located in the south central portion of the San Joaquin Valley and has a service area of approximately 340,000 acres in Tulare and Kings Counties. The mission of the District is to conserve and store waters of the Kaweah River, provide flood control, conserve and protect groundwater, and ensure adequate and reliable water supplies for the Kaweah River Basin.

As part of its groundwater management activities, the District estimates and reports groundwater pumpage over time for six individual hydrologic units. Historically, due to lack of groundwater production monitoring at individual irrigation wells, the District has estimated pumpage using a combination of estimated annual crop acreages and crop evapotranspiration (ET) estimates based on published crop coefficients and reference ET from nearby weather stations. These values are used to estimate the total applied water demand in each hydrologic unit, with pumpage calculated as the portion of the applied water demand not met by measured surface water applications. This approach leads to uncertainty in estimated pumpage due to a number of factors including:

  • Infrequent update of available data describing changes in cropping and acreages by hydrologic unit
  • Difficulty identifying double- and triple-cropping via ground-based crop surveys
  • Differences in crop timing and intensity from year to year due to weather and market factors
  • Differences between idealized conditions for which crop coefficients were developed and actual growing conditions
  • Difficulty estimating the portions of total applied irrigation water consumed as crop ET or lost to deep percolation and tailwater

In order to help overcome these challenges and improve the confidence in estimates of consumptive use and applied irrigation water within Kaweah Delta Water Conservation District, the District hired Davids Engineering to perform a combination of remote sensing and root zone water balance analyses to develop spatially distributed time series estimates of crop ET, applied water demand, and other fluxes of water for the twelve-year period from 1999 to 2010.

SEBAL 2007 ETa, KDWCD and Southern San Joaquin Valley.

Initially, an analysis of spatially distributed actual crop evapotranspiration (ETa) was performed using the Surface Energy Balance Algorithm for Land (SEBAL®) for the 2007 calendar year. SEBAL quantifies surface energy fluxes at the Earth’s surface including net solar radiation (Rn), soil heat flux (G), and sensible heating (H).  Then, based on the principle of conservation of energy, the latent heat flux (L) is calculated, which is directly proportional and readily converted to ETa. SEBAL has been extensively validated in the U.S. and worldwide over more than 15 years and has been found to consistently provide estimates of ETa that agree within 5% of reliable ground-based estimates on a seasonal or annual basis. Additional information describing the SEBAL model and its validation is available at www.sebal.us.

SEBAL results were combined with detailed cropping information developed by the California Department of Water Resources for 2007 to estimate local crop coefficients for 2007. These results were then compared to standard published values used in California. Based on the comparison, important differences were observed.

In order to avoid the relatively high costs of estimating spatially distributed ETa on an annual basis using SEBAL, a simplified remote sensing approach was developed using a combination of regular estimation of the amount of green vegetation present using the normalized difference vegetation index (NDVI) and a daily rootzone water balance model. NDVI imagery was acquired at monthly or more frequent intervals for the twelve-year analysis period and used to develop a daily time series of the basal crop coefficient, which describes crop transpiration, for each field within the District.

Sample daily model results for walnut orchard

A daily root zone water balance model was developed based on the procedures described in FAO Irrigation and Drainage Paper No. 56 for the dual crop coefficient approach. This model was implemented in a database environment and used to estimate inflows to and outflows from each field from applied irrigation water and precipitation. Model results for an independent set of fields were used to validate the approach, and it was found that total ET estimated based the combination of NDVI imagery and the daily root zone water balance model agree with SEBAL results within 1% over a three year period.

This approach to quantifying ETa and applied water demand at a highly discrete spatial and temporal scales over a multi-year time period has resulted in increased reliability in estimates of total pumpage and changes in groundwater storage by reducing reliance on cropping data, inherently accounting for changes in crop timing and intensity from year to year, and inherently accounting for differences between actual growing conditions and the idealized conditions upon which many published crop coefficients were developed. Because of the relatively low cost of running the NDVI-based model, the District is exploring the possibility of incorporating annual updates into its analysis and reporting processes.