Estimation of supplementary water capacity for non-irrigated paddy fields during the dry season in northeast Thailand

This study estimated the supplementary water capacity for paddy fields by estimating the water supply in each province during the dry season. Northeast Thailand was selected for this analysis because of the availability of agricultural statistical data distinguishing irrigated rice fields and non-irrigated rice fields during the dry season. This region is in Monsoon Asia, and so the climate has two seasons: the dry season and the wet season. Since about 70% of the annual rainfall is concentrated in the wet season, rice cannot be produced using only the dry season precipitation. In other words, farmers must obtain water from some other source to supply water to their paddy fields when growing rice in the dry season. Water is supplied from dam reservoirs in the irrigated rice fields, and water is supplied to the non-irrigated rice fields by gravity and small pumps from small ponds and canals. Thus, it was assumed that the amount of water supplied to paddy fields during the dry season can be calculated by subtracting rainwater during the dry season from the amount of water used in paddy fields. Based on rice production data, the values of water resources required to produce a unit weight of rice and rainwater, attempts were made to estimate the supplementary water capacities to paddy fields from the estimated supplied water and its maximum values in each province. Because there is no data for small ponds and canals for non-irrigated paddy fields in each province, the validity of the values estimated from the agricultural statistics was examined by comparing the supply water capacities for irrigated paddy fields estimated from the statistical data with those calculated from the Royal Irrigation Department data for large- and medium-scale reservoirs in each province. As a result, both differences tended to be less than 1 mm in the provinces whose harvested area of irrigated rice was small. It was therefore considered that supplementary water capacities to the non-irrigated paddy fields in each province can be estimated by the same method as for the irrigated paddy fields because it was assumed that the actual situation was similar to the non-irrigated paddy fields even if the classification of the rice fields was "irrigated". As a result, the supplementary water capacity tended to be high in the provinces through which the Chi River flows and the province upstream of the Mun River.