Partitioning of Evapotranspiration of Irrigated Wheat as Affected by Rice Straw Mulch.

Soil evaporation is considered to be a non-beneficial loss of water, aside from its effect in moderating vapour pressure deficit on crop water use. Therefore, measures to reduce soil evaporation such as mulching are of interest for their potential to reduce water losses and increase crop water productivity. In the intensive irrigated rice-wheat systems of the North-West Indo-Gangetic Plains, rice residues are normally burnt prior to wheat sowing. However, recent machinery developments enable simultaneous direct drilling and mulching of wheat in residues of the previous rice crop. Field experiments were conducted during 2006-07 and 2007-08 in Punjab, India on a silt loam soil to investigate the effect of rice straw mulch on components of water balance of a succeeding wheat crop. Daily soil evaporation was measured using mini-lysimeters (open PVC cylinders 20 cm long, 10 cm diameter) throughout the cropping season, and total seasonal Evapotranspiration was estimated from the water balance using measurements of irrigation, rainfall and soil water depletion. Irrigations (75mm) were scheduled according to cumulative pan evaporation-i.e. both treatments were irrigated on the same day. Average daily soil evaporation with and without mulch was 0.68 and 0.90 mm day^-1, respectively, in 2006-07 and 0.88 and 1.09 mm day^-1 in 2007-08. The mulch lowered total soil evaporation by 34.7 and 32.2 mm each season, and much of this appeared to be partitioned into transpiration which increased by 30.5 and 28.1 mm in 2006-07 and 2007-08, respectively. The additional water transpired by the mulched crop increased grain yield significantly in 2006-07 (610 kg ha^-1), but there was no effect on grain yield in 2007-08. There was trend for mulch to lower transpiration water productivity; 19.4 and 19.1 kg ha^-1.mm without and with mulch, in 2006-07 and 16.8 and 15.4 kg ha^-1 .mm, in 2007-08, respectively.