Submitted to: Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2012
Publication Date: 2/6/2012
Citation: Johnson, L.F., Trout, T.J. 2012. Satellite-assisted monitoring of vegetable crop evapotranspiration in California's San Joaquin Valley. Remote Sensing. 4:439-455. Interpretive Summary: Precise scheduling of irrigation applications is important for efficient water use which is critical as water supplies for agriculture decline. Remote sensing can be used to assess the condition and water needs of crops. Data collected in the San Joaquin Valley was used to show that crop water requirements is related to crop ground cover, and that crop ground cover can be closely estimated with satellite images. When this information is combined with reference evapotranspiration measurements from ground-based weather station networks, good estimates can be made of crop water use. This information can be made available on-line to growers to facilitate improved irrigation scheduling.
Technical Abstract: Reflective bands of Landsat-5 Thematic Mapper satellite imagery were used to facilitate the estimation of basal crop evapotranspiration (ETcb), or productive water use, in San Joaquin Valley during 2008. A ground-based digital camera measured green fractional cover (Fc) of 49 commercial fields planted to 18 different crop types (row crops, grains, orchard, vineyard) of varying maturity over 11 Landsat overpass dates. Landsat L1T terrain-corrected images were transformed to surface reflectance and converted to normalized difference vegetation index (NDVI). A strong linear relationship between NDVI and Fc was observed (r2=0.96, rmse=0.062). The resulting regression equation was used to estimate Fc for crop cycles of broccoli, bellpepper, head lettuce, and garlic on nominal 7-9 day intervals for several study fields. Prior relationships developed by weighing lysimeter were used to transform Fc to fraction of reference evapotranspiration, also known as basal crop coefficient (Kcb). Measurements of grass reference evapotranspiration from the California Irrigation Management Information System were then used to calculate ETcb for each overpass date. Temporal profiles of Fc, Kcb, and ETcb were thus developed for the study fields, along with estimates of seasonal water use. Good agreement was seen with FAO-56 irrigation guidelines for Kcb maximum value and range, while the satellite observations revealed differences associated with crop phenology.