Quantifying evaporation and transpiration in field lysimeters using the soil water balance

[摘要] The main aim of this study was to determine the transpiration efficiency coefficient(TEC) for three C3 crops; canola, wheat and lucerne. TEC relates to the efficiency ofwater management in crop production. It is defined as the ratio of seed or biomass to theproduct of transpiration and vapour pressure deficit. Of these variables, transpiration isthe most difficult to measure. Two experiments (canola, 2007 and wheat, 2007&2008)were therefore designed with the aim of partitioning evapotranspiration (ET) into itscomponents of evaporation (E) from the soil and transpiration (T) from the plant. Theseexperiments were based on a split plot design, with two soils (Clovelly and Bainsvlei)and two surface treatments which comprised of a bare soil for measuring ET and a 50mm thick gravel mulch for measuring T using the lysimeter unit of the University of theFree State at Kenilworth near Bloemfontein. These components were measured regularlyand E was derived by subtracting T from ET. The results showed that for canola, E was12% of the total ET (809 mm) and for wheat E was 27% of total ET (639 mm). Thepercentage contribution of T to ET was high in both crops: 718 mm or 88% of total ET ofcanola and 489 mm or 63% of total ET of wheat. Conclusive evidence showed that cropsshould be managed differently with respect to their individual irrigation water demands.The remaining three experiments were dedicated to factors influencing the TEC of crops.Specific objectives were to establish the effect of growth periods during the reproductivestage on the TEC of canola, the effect of weather on the TEC of wheat and effect ofcutting periods on the TEC of lucerne. All experiments were conducted in the lysimeterunit and measurements were based on the soil water balance of both soils. TEC wasexpressed as grain yield (GY) or seed yield (SY), above-ground biomass (AGB) and totalbiomass (TB). Soils had no significant effect on TEC. However, TEC of canola wassignificantly affected by growth periods. For growth periods, TECABG varied between3.82 and 4.95 g kPa mm-1 and TECTB between 3.94 and 5.04 g kPa mm-1. For wheat itwas concluded that weather had no influence on the TEC based on AGB, but TEC basedon GY was significantly lower in 2008 (TEC = 0.9 g kPa mm-1) compared to 2007 (TEC = 2.3 g kPa mm-1). This was caused by severe frost which occurred in the earlyreproductive stage. The result revealed a mean TECAGB of 4.75 g kPa mm-1 for the twowheat seasons. The results on lucerne suggested that cutting periods do played asignificant role in the TECAGB of the crop. TEC decreased from 3.86 g kPa mm-1 for thefirst cutting period to 2.22 g kPa mm-1 for the sixth cutting period, with a mean TECvalue of 2.84 g kPa mm-1 for all six cutting periods. TEC values for canola, wheat andlucerne in this study are consistent with values reported for other C3 crops in the semiaridenvironments and are therefore recommended for use in models.

[发布机构] University of the Free State