Thesis (M.Eng.) - Asian Institute of Technology, 1980

A field experiment was conducted in Lam Pao Irrigation Project on plots with five different water table levels , 52, 87 , 104 , 134 , and 277 cm depth labelled WTDl , WTD2 , WTD3 , WTD4 , and WTD5, respectively. The water table was allowed to fluctuate wit h the field condition , and dropped throughout the period of experiment , as much a s 40 cm in WTDl treatment . Three level s of irrigation were designated as Il, I2, and I3 corresponding to 0 , 0.6 1 1 . 2 time the evaporative demand . Three levels of fertility were designated as Fl, F2 and F3 corresponding to 0 , 0 . 6 , 1.2 time the basic fertilizer rate. Irrigation was applied on the basis of 10 days interval. The soil moisture depletion was measured on the basis of one day before and two days after irrigation by Neutron probe. Based on the soil moisture depletion study within the root zone, the water requirements of sweet corn were found to be 36.0 cm or 4.8 mm/day. The ratio of ET/Epan of F3I3 treatment ranged from 0 . 48 to 0 . 76, The soil moisture depletion of F3I3 indicated no significantly difference among WTD2 ,WTD3 , WTD4 and WTD5, leading to the conclusion that no ground water was supplied to plants in those plots. For WTDl- plot with shallowest water table, the groundwater~ contribution to the evapotranspiration of sweet corn was estimated about 15.0 cm. In contrast, when assumed the soil moisture depletion occurred from the soil surface to the capillary rise level, the water requirements were found 'to be 50.3 cm or 6.6 mm/day . The groundwater· contribution to the evapotranspiration of-WTDl , WTD2 , WTD3 and WTD4 was 32.4 1 12 . 2 and 9.8 cm, respectively, for F3I3 treatment. No salinity problem was encountered in the experimental site. Maximum yield of 3,260 kg/ha was found at F3I3 of WTD3 - plot. As the inputs of water and fertilizer increased, the yield response of sweet corn increased and reached a maximum at F3I3, with an exception of WTD2, which the maximum yield response was at F2I2 treatment . The explanation for very low yield is due t o very poor soil structure. A high bulk density of 1.7 gm/cc limits vertical root development. Water deficits occurred during the critical period, even in the I3- treatment. Water application efficiency showed that at the I3-treatment, the amount of water applied exceeded the maximum holding capacity of soil and losses occurred as deep percolation. To reduce this loss, the amount of water applied should be approximately 0.84 times evaporative demand.