The Upper Limit of Available Water for Kamphaeng Saen Soil Series

Abstract

Available water capacity is a widely used soil parameter in the evaluation of soil productivity potential and irrigation planning. However, the upper limit is never unequivocally defined and agreed upon. It is proposed in this study that three aspects of soil behavior must be considered in setting the upper limit. These are the amount of water that can be released from the soil matrix, the adequacy of soil aeration and the rate of water loss from the root zone. Differential water capacity, which is obtained as the slope of soil water characteristic curve, is used to indicate how much water is released from the soil when matric potential drops. Soil aeration is considered as not limiting when the air pore space does not fall below 0.1 of total soil volume. Drainage loss is determined from internal drainage study. Matric potential of soil profile was monitored for several days after the profile was wetted initially and subsequent free drainage was allowed with no evaporation loss from soil surface. Kamphaeng Saen soil series (Ks, fine silty, mixed Udic Haplustalfs) was chosen as a case study. Imposing the above three aspects, it was found that soil water content at matric potential of -0.005 MPa met at the criteria and should be used as the upper limit of soil water availability. Water at this potential has high value of differential water capacity and 0.11 m³m^-3 air space, and was attained in the profile 2 days after internal drainage started. Drainage loss from 1.00 m profile was about 3-4 mm d^-1, which is mostly below the potential evapotranspiration rate of Kamphaeng Saen area. Therefore, drainage loss is considered as minimal in comparison to daily water use by crops. The water content at upper limit of Ks soil series in 0.39 m³m^-3. When the lower limit is set to the conventional matric potential of -1.5 MPa, the corresponding water content is 0.195 m³m^-3. The resulting available water capacity for the 1.00 m profile of Ks soil series is 0.195 m³m^-3. This amounts to 118 mm of water for a root zone of 0.50 m and 195 mm for 1.00 m root zone.