|Hunsaker, Douglas - Doug|
|Pinter Jr, Paul|
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2007
Publication Date: 12/20/2007
Citation: Hunsaker, D.J., Fitzgerald, G.J., French, A.N., Clarke, T.R., Ottman, M.J., Pinter Jr, P.J. 2007. Wheat irrigation management using multispectral crop coefficients: crop evapotranspiration prediction. Transactions of the ASABE. 50(6):2017-2033 Interpretive Summary: Reliable techniques are needed to help farmers know when to irrigate and how much water to apply. Evapotranspiration, or ET, is the measurement of crop water use and provides the information needed for efficient irrigation. A method widely used by irrigation managers to estimate the crop ET during the growing season multiplies reference ET data, available from local weather station networks, by standard crop coefficients (Kc). Unfortunately, the crop ET calculated with the standard Kc data that are available is often not accurate for irrigation scheduling and efficient water use. Large errors in the calculated ET arise because actual Kc varies with the climate, the growth rate of the crop, and with specific farmer practices. This research tested a technique with wheat aimed at correcting the Kc depending on how the crop actually developed during the season. The technique used frequent remote sensing observations of wheat canopy reflectances to continually correct the Kc during the season. Results indicate that the remote sensing technique produced more precise estimates of measured ET than the standard Kc for a wide range of wheat growth conditions. Application of this technique should provide more accurate Kc and crop ET information, which would help growers determine optimal irrigation scheduling strategies. This research will be of interest to irrigation farm managers, irrigation consultants, government agencies, and the irrigation industry.
Technical Abstract: A method widely used for irrigation management determines crop evapotranspiration (ETc) from reference evapotranspiration (ETo) calculations and estimated crop coefficients. However, standard time-based crop coefficients may fail to represent the actual crop water use, for example, when deviations in weather or agronomic constraints appreciably change crop development patterns from typical conditions. In this study, the FAO-56 dual crop coefficient procedures were applied during experiments with wheat to calculate the estimated ETc for irrigation scheduling. The objective for this paper was to determine whether basal crop coefficients (Kcb) determined from a normalized difference vegetation index (NDVI treatment) improve the prediction of ETc over a standard application with a locally developed time-based Kcb curve (FAO treatment). The experiments conducted for two seasons in central Arizona included sub-treatments, equally replicated within the NDVI and FAO treatments, of three plant densities (typical, dense, and sparse) and two nitrogen levels (high and low) to provide a range of crop development and water use conditions. The effects of plant density and N level resulted in significant differences in measured seasonal ETc. Large variations that occurred in the observed Kcb and ETc patterns between sub-treatments were better correlated with the NDVI than the FAO treatment. The mean absolute percent difference for predicted ETc was significantly smaller for NDVI than FAO during both seasons. The treatment difference was 5% for the first season, but 10% for the second season when an unexpected early decline in ETc and Kcb was effectively predicted by the NDVI treatment but not by the FAO treatment. NDVI appears to be a more robust approach for Kcb estimation of wheat, able to reliably predict actual ETc for both typical and abnormal water use conditions.