Evaluating the crop coefficient using spectral reflectance

Agronomy Journal
By: , and 



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 Kc and crop growth are established, a mean Kc 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.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Evaluating the crop coefficient using spectral reflectance
Series title Agronomy Journal
DOI 10.2134/agronj1982.00021962007400060010x
Volume 74
Issue 6
Year Published 1982
Language English
Publisher American Society of Agronomy
Contributing office(s) Earth Resources Observation and Science (EROS) Center
Description 5 p.
First page 967
Last page 971
Online Only (Y/N) N
Additional Online Files (Y/N) N