Crop evapotranspiration — a technique for calculation of its components by field measurements

Abstract

A simple technique is described whereby standard field measurements of crop evapotranspiration (ET), evaporation from an uncropped soil (ES), green area index (G), the crop extinction coefficient (K) and above ground dry matter production (TDM) are manipulated to compute the seasonal variation in crop transpiration (T), evaporation from the soil beneath the crop (ESC) and transpiration efficiency (TE). The technique is illustrated using data from a trial in which wheat (c.v. Mexipak) and barley (c.v. Beecher) were grown with and without nitrogen and phosphate fertilizer at two locations in Northern Syria, namely Tel Hadya and Breda.

T values were very low during the cool winter months, reflecting poor interception of radiant energy, and ESC accounted for nearly 100% of ET during this period. In early spring, as G increased, T values also increased and reached maximum values around anthesis of 3.7 and 3.4 mm/day for fertilized and unfertilized wheat at Tel Hadya, and 2.0 and 1.7 mm/day for fertilized and unfertilized barley at Breda. ES values dropped correspondingly. In the post anthesis period, due to greater soil moisture depletion during the pre-anthesis period, fertilized crops had lower T values and this was reflected in both plant water status and grain weight measurements. During this period, as moisture stress increased and leaf senescence occurred, T values dropped sharply and SC values rose. On a seasonal basis, T acounted for 68 and 61% of ET in thefertilized and unfertilized wheat and 50 and 38% of ET for the fertilized and unfertilized barley.

The pattern of seasonal variation in TE was influenced, in the preanthesis period, by the absence of root dry matter measurements, but reached values between 65 and 75 kg/ha/mm around anthesis when root growth ceased. TE values were inversely related to vapour pressure deficit (represented by E0) in the post anthesis period and corresponded well to predicted values. On a seasonal basis, TE values were very similar across crop, treatment and location ranging between 42.8 and 45.7 kg/ha/mm.