Soil heat flux under irrigated, partially irrigated wheat and bare vertisol in relation to varying soil moisture and leaf area index at Pune

Abstract

Soil heat flux as a function of moisture content in the drying cycles both under irrigated and partially irrigated wheat and in bare black vertisol was studied to relate temporal changes in soil heat fluxes to changes in leaf area index (LAI), soil water content and meteorological parameters. Moisture content dropped as low as 1 to 5 percent by weight at surface layer and 5 to 10 percent by weight at subsurface layers when irrigation was shut off under partially irrigated plot. The peak weekly mean global, net and reflected radiation during the crop growing season in this semi-arid region goes as high as 655, 361 and 179 Wm^-2 respectively in the year 1996-97 and 700, 366 and 137 Wm^-2 respectively during the year 1997-98. Soil temperature at 1430 hours IST ranged from 27 to 40°C at the surface   (5.0 cm) layer under irrigated condition. It was 7 to 10° C and 2 to 3° C higher in the surface and subsurface layers respectively under partially irrigated wheat during the drying cycle. The upward soil heat flux ranged from -3.3  to  -29.7 Wm^-2 at 0730 hours IST, whereas downward heat flux at 1430 hours IST ranged between 8.4 to 33.9 Wm^-2 under irrigated condition. Under partially irrigated condition it varied from -0.7 to -9.2 Wm^-2 at 0730 hours IST and from 10.4 to 20.9 Wm^-2 at 1430 hours IST; whereas it varied from -4.2 to -17.6 Wm^-2 at 0730 hours IST and 6.7 to 24.7 Wm^-2 at 1430 hours IST, under bare soil. The maximum mean heat fluxes into 5-20 cm soil depth under irrigated, partially irrigated and the bare soil were 10.2, 5.0 and 6.5 Wm^-2 and 9.2, 7.1, and 6.7 Wm^-2 respectively in the years 1996-97 and 1997-98.  Soil temperature at 1430 hours IST both for irrigated wheat and bare vertisol and global radiation with soil temperature at 0730 hours IST under partially irrigated wheat were found to be the best predictors of soil heat flux. The relationships between soil heat flux and soil water content at different depth intervals along with LAI and other meteorological parameters could be used for estimation of soil heat flux, both for irrigated and partially irrigated wheat and bare soil.