Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 10/1/2010
Publication Date: 12/13/2010
Citation: Johnson, L., Trout, T.J., Wang, D., Melton, F. 2010. Landsat-based monitoring of basal crop coefficients in the San Joaquin Valley. American Geophysical Union 2010 Fall Meeting, Dec 13-17, 2010, San Francisco, CA. Interpretive Summary:
Technical Abstract: Fresh water resources are becoming increasingly scarce in California due to urbanization, environmental regulation, and groundwater depletion. The strain is projected to worsen under various climate change scenarios and is exacerbated by declining water delivery infrastructure. It is estimated that irrigated agriculture currently commands more than 70% of the state’s water supply, and many growers are striving to improve water use efficiency in order to help maintain the state’s rich agricultural heritage. Remote sensing technology offers the potential to monitor cropland evapotranspiration (ET) regionally, while making farm-based irrigation scheduling more practical, convenient, and possibly more accurate. Landsat5-TM imagery was used in this study to monitor basal crop evapotranspiration (ETcb), which is primarily related to plant transpiration, for several San Joaquin Valley fields throughout the 2008 growing season. A ground-based digital camera was used to measure fractional cover of 48 study fields planted to 18 different crop types (row crops, grains, orchard, and vineyard) of varying maturity over 12 dates coinciding with Landsat overpasses. Landsat L1T terrain-corrected images were atmospherically corrected to surface reflectance by an implementation of the Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS), then converted to normalized difference vegetation index (NDVI) on a per-pixel basis. A strong linear relationship between NDVI and fractional cover was observed (r2=0.96), and a resulting conversion equation was used to transform all imagery to fractional cover. Conversion equations previously developed by use of weighting lysimeters were then used to transform fractional cover to basal crop coefficient (Kcb; ratio of crop transpiration plus a small diffusive soil evaporation component to reference ET). Finally, measurements of grass reference ET (ETo) from the California Irrigation Management Information System were used to retrieve ETcb (mm/d) for each overpass date. Spatially contiguous maps of Kcb and ETcb were generated for the Valley for each satellite scene. Temporal profiles of Kcb and ETcb were developed for several individual study fields located in the Landsat path 42/43 overlap zone, providing a revisit period of approximately 8 days. Two sets of ETcb profiles were produced - one referenced to 2008 ETo (representing actual conditions on overpass date) and another to long-term historical average ETo. The satellite-based approach, as implemented in regions with an available ETo network, potentially enables timely estimation of crop water use for resource monitoring and scheduling of irrigation events.