Estimating crop coefficients from fraction of ground cover and height

Abstract

The FAO-56 procedure for estimating the crop coefficient K c as a function of fraction of ground cover and crop height has been formalized in this study using a density coefficient K d. The density coefficient is multiplied by a basal K c representing full cover conditions, K cb full , to produce a basal crop coefficient that represents actual conditions of ET and vegetation coverage when the soil surface is dry. K cb full is estimated primarily as a function of crop height. K cb full can be adjusted for tree crops by multiplying by a reduction factor (F r) estimated using a mean leaf stomatal resistance term. The estimate for basal crop coefficient, K cb , is further modified for tree crops if some type of ground-cover exists understory or between trees. The single (mean) crop coefficient is similarly estimated and is adjusted using a K soil coefficient that represents background evaporation from wet soil. The K c estimation procedure was applied to the development periods for seven vegetable crops grown in California. The average root mean square error between estimated and measured K c was 0.13. The K c estimation procedure was also used to estimate K c during midseason periods of horticultural crops (trees and vines) reported in the literature. Values for mean leaf stomatal resistance and the F r reduction factor were derived that explain the literature K c values and that provide a consistent means to estimate K c over a broad range of fraction of ground cover.

Crop coefficient approaches based on fixed estimates of leaf resistance are not appropriate for estimating water use of citrus

“…(Table 4). In the summer rainfall region, despite large differences in average daily ET o during the year, the average K t for both orchards remained relatively constant throughout the year, which agrees with the single crop coefficient for citrus suggested by Allen and Pereira (2009). However, in the winter rainfall region there was a significant difference in K t values between the two seasons, with the winter K t being significantly larger than the summer K t , as observed by García…”

“…The relatively simple and convenient crop coefficient approach of Allen et al (1998) relates water use of various crops to evapotranspiration from a well-watered hypothetical short grass reference surface. K t values for the measurement orchards were initially derived using parameters for citrus provided by Allen and Pereira (2009), where the value for M L was set to 1.5, apparent effective r l for the initial and midseason periods was 420 s m -1 and r l at the end of the season was 150 s m -1 , and measured canopy dimensions and weather data were used. However, these transpiration coefficients (K t Allen and Pereira) did not compare well with the measured K t values from the three orchards, and were at times in the season higher than the FAO-56 K cb values for citrus given by Allen and Pereira (2009) (Fig.…”

“…where ƒ c eff is the effective fraction of ground covered or shaded by vegetation [0.01-1] near solar noon, M L is a multiplier on ƒ c eff and is an attempt to simulate hydraulic resistances within the plant and is expected to range between 1.5 and 2.0, with a value of 1.5 recommended for citrus (Allen and Pereira 2009). ƒ c eff was calculated as the ratio of tree canopy width to inter-row spacing or the ratio of ground shaded area by the crop at solar noon to the total area available to the tree, following Allen et al (1998) in the "Delta" Valencia orchard with a north-south orientation.…”

“…In an attempt to make crop coefficients more transferrable between different orchards, Allen and Pereira (2009) Villalobos et al (2013), were chosen as K cb includes some evaporation when the soil surface is dry and only transpiration was measured in this study.…”

Response of bell pepper crop subjected to irrigation depths calculated by different methodologies

“…ETc was estimated using single-Kc (Mantovani & Costa, 1998) and dual-Kc (Allen et al, 2006;Allen & Pereira, 2009;Freitas et al, 2017) methodologies, according to Eqs. 1 and 2, respectively.…”