Drying of some Philippine and Indonesian puddled rice soils following surface drainage:: Numerical analysis using a swelling soil flow model

Heavy clay soils used for paddy rice production in SE Asia are usually puddled to grow the rice crop. Soil conditions following puddling and drying might be unsuitable for following dry season crops, such as mungbean or soybean, which often produce poor or erratic yields. We analysed water movement in the drying process in order to better understand the drying behaviour in order that we may generalise the findings, and to examine the prospects for using a swelling soil flow theory in predicting that behaviour. A numerical solution to a general, material coordinate-based, one-dimensional flow equation for swelling soils was used to analyse experimental results from four sites in Indonesia and the Philippines. The soils at the four sites were a Ustic Epiaquert, a Typic Ustropept, an Aeric Tropoquept and a Chromic Epiaquert. In the experiments, evaporation, cracking and changes to moisture ratio (ratio of volume of moisture to volume of solid) and void ratio were monitored for several weeks following surface drainage. Hydraulic properties were also measured. merical predictions of changes to moisture ratio compared well to the measured results. Predictions of cracking were based on assumptions about the ratio of vertical to lateral shrinkage, and reasonable comparisons were obtained with the measurements. The analysis showed that the measurement of soil evaporation by mini-lysimeters provided a good estimate of the overall soil evaporation at the wet end but underestimated evaporation from dry soil. The analysis also showed that the measurements of void ratio in the experiments were sometimes in error, because of difficulties in accurate volume sampling of very weak, wet soils. It is shown that in these swelling systems it was better to use material coordinates and moisture ratios rather than spatial coordinates and volumetric moisture contents.