Abstract
Crop diversification means introducing non-rice crops to paddy based land during the dry season in Asian developing countries. In this diversified land, moisture movement in subsurface soil layers is complex and diverse for the existence of plow sole formed through paddy cultivation. This complex movement influences estimation of crop water consumptive use (CU) by the soil moisture depletion method. To analyze the complex moisture movement, the author applied total potential (TP) gradients, CU and soil moisture extraction pattern (SMEP). The CU and SMEP were determined by the modified soil moisture depletion method which estimated evapotranspiration (ET) distinguishing ET from drainage loss in the depletion amount with the zero flux plane (ZFP) identified from the TP gradients. The results of analysis are summarized below.
(1) No influence is identifed for existence of plow sole to estimate CU amounts.
(2) The CU rate of plow sole layer is bigger than that of the right above layer.
(3) Upward water supply from the subsurface soil layer through plow sole is apparently intermittent in the case of long irrigation intervals.
(4) Under low evaporative demand conditions, TP gradients become sharp above the plow sole although upward water supply occurs from the layers below the plow sole.
(5) Under high evaporative demand conditions, TP gradients become sharp in and above the plow sole layer, mainly to supplement upward water supply which is decreased for the lowered hydraulic conductivity of the plow sole with soil dry.
(6) Under extremely high evaporative demand conditions, a phenomena happens that the TP gradient between the plow sole and the right above layer shows adverse moisture movement direction to the real flux.
For the adverse TP gradient described in (6), an untrue ZFP appears above the plow sole layer. Analyzing TP gradient changes is, thus, required to avoid under estimation of CU for the untrue ZFP in applying the modified soil moisture depletion method under high evaporative demand conditions.
This study is based on the data collected in a Japan's technical assistance project called Diversified Crops Irrigation Engineering Project which was carried out from 1987 to 1992 in the Philippines.
