Abstract
The layer of soil from the surface to a depth of 1 m was monitored using a soil profile probe (Delta-T Devices Ltd. PR-1) at 76 points in the dry season from November 18, 2005 to February 8, 2006. The study sites comprised 2 small watersheds in the Khon Kaen province of Northeast Thailand. The dry season began from October 13 but there was a total of 30 mm of irregular rainfalls in early November. Rainfall greater than 1 mm was not observed during the monitoring period. The average soil-stored water was 132 mm at the beginning of December and 109 mm at the beginning of February. The decrease in soil moisture was 23 mm. This was much less than the potential evaporation of 324 mm during this period. The amount of soil moisture was different at each monitoring point. The effects of the topographical level of the watersheds were not clear for both the talweg and cross line. On the contrary, the type of vegetation affected the soil moisture. The moisture level in December was 25 mm in forest, 79 mm in fallow uplands (weeds), 96 mm in cassava fields, 131 mm in sugarcane fields, 147 mm in fallow paddy fields (weeds), and 163 mm in after-rice (little vegetation) fields. The decrease of the soil stored water between December and February was as follows: 3 mm in forest, 19 mm in fallow uplands (weeds), 30 mm in cassava fields, 26 mm in sugarcane fields, 28 mm in fallow paddy fields (weeds), and 18 mm in after-rice (little vegetation) fields. The soil moisture at each layer increased with depth. However, the soil moisture in forest was an exception; it decreased from the soil surface to a depth of 1 m. This implies that the available soil moisture had already evaporated by December. The 22 mm of soil moisture in the forest can be considered as unavailable water. In December, the available water in the after-rice (little vegetation) fields was estimated to be 141 mm. The transpiration coefficients are known to be approximately 100 for corn and 200~500 for general crops. Therefore, 141 mm of water can yield a maximum dry matter of 1.4 kg·m–2 for corn and 0.7~0.3 kg·m–2 of general crops.
