FAO-56 Dual Crop Coefficient Method for Estimating Evaporation from Soil and Application Extensions

Abstract

Crop coefficient curves provide simple, reproducible means to estimate crop evapotranspiration (ET) from weather-based reference ET values. The dual crop coefficient ͑K c ͒ method of the Food and Agricultural Organization of the United States (FAO) Irrigation and Drainage Paper No. 56 (FAO-56) is intended to improve daily simulation of crop ET by considering separately the contribution of evaporation from soil. The dual method utilizes "basal" crop coefficients representing ET from crops having a dry soil surface and separately predicts evaporation from bare soil based on a water balance of the soil surface layer. Three extensions to the evaporation calculation procedure are described here that are intended to improve accuracy when applications warrant the extra complexity. The first extension uses parallel water balances representing the portion of the soil surface wetted by irrigation and precipitation together and the portion wetted by precipitation alone. The second extension uses three "stages" for surface drying and provides for application to deep cracking soils. The third extension predicts the extraction of the transpiration component from the soil surface layer. Sensitivity and analyses and illustrations indicate moderate sensitivity of daily calculated ET to application of the extensions. The dual K c procedure, although relatively simple computationally and structurally, estimates daily ET as measured by lysimeter relatively well for periods of bare soil and partial and full vegetation cover.

Nitrogen response and water use efficiency of sweet sorghum cultivars

“…In case of irrigated sorghum crop, as per the general guidelines, the values for K cb during initial, mid-season and at end of season are 0.15, 1.10, and 0.64 (Allen et al, 1998). The evaporation component in ET c is estimated by following three stage method described by Allen et al (2005). Basal crop coefficient gives potential transpiration (Tp) when the water available for plant uptake is not limited.…”

Hydroecological model predictions indicate wetter and more diverse soil water regimes and vegetation types following floodplain restoration

“…Rainfall inputs were determined by measurements collected at an on-site weather station (Onset Computer Corporation, Bourne, Mass.). The meteorological data were also used to determine potential evapotranspiration based on the FAO-56 Penman-Monteith method [Allen et al, 2005].…”

“…Root water uptake via transpiration is represented as a sink term that is applied using a normalized root density function [Zuo et al, 2004] over a rooting depth of 0.5 m as discussed in Booth and Loheide [2010]. Potential evapotranspiration was determined and partitioned into potential evaporation and potential transpiration using leafarea index (LAI) and following the FAO-56 PenmanMonteith dual crop coefficient method [Allen et al, 2005]. Meteorological inputs for the potential evapotranspiration model and precipitation were measured by an on-site weather station (Onset Computer Corporation, Bourne, Mass.).…”

Can a change in cropping patterns produce water savings and social gains: A case study from the Fergana Valley, Central Asia

“…The irrigation regime for winter wheat was the same as for the previous scenario, while for green gram we implemented the irrigation scheme considered by Webber (2008). Sumochkina et al (1977), Webber (2008), Allen et al (1998Allen et al ( , 2005, Smeal et al (1991), and Sommer et al (2013). Total irrigation applications for the winter wheat/green gram system were 652-847 mm under FI, 587-762 mm under LDI, 503-672 mm under HDI, and 423 mm for rainfed winter wheat followed by the irrigated green gram.…”