A field study was conducted in four differentially irrigated plots of alfalfa (Medicago sativa L.) planted in Shiprock sandy loam (coarseloamy, mixed, mesic Typic Haplargid) to assess spectral reflectance for estimating the crop coefficient (Kc), defined here as the ratio of actual to potential evapotranspiration (ET). A bidirectional reflectance factor was measured using a three-channel (0.63 to 0.69 µm, 0.76 to 0.90 µm, and 1.55 to 1.75 µm) handheld radiometer, and was used to calculate a perpendicular vegetation index (PVI). Actual ET was determined by the water balance method in non-weighing lysimeters, and potential ET was calculated using Penman's equation.

Significant linear relationships were found between PVI and percent cover (r² = 0.911), and between Kc and percent cover (r² = 0.815). In addition, the position of the PVl intersection on the soil background line changed as a result of soil moisture increases following irrigation, even at high percent cover. Thus, once experimental relationships between K_c and crop growth are established, a mean K_c can be determined from spectral estimates of stage of development and the soil background component of PVI can be used to adjust the mean K, for increased evaporation following irrigation because the ratio of actual to potential evapotranspiration will approach 1 when the soil surface is wet.