A field study was carried out in the Cukurova Region, Southern Turkey to investigate the magnitude of the components of water balance, and the water uptake by cotton roots in relation to hydraulic properties of a clay soil. A plot cropped with cotton and with bare soil only were equipped with tensiometers, gypsum blocks, and access tubes for neutron probe to monitor soil water potential and water content.
The hydraulic conductivity values, evaporation and drainage rates, and water withdrawal of roots were determined from field data with numerical calculations based on water flow equations.
Results showed that the evaporation from bare soil was generally high during the three month period May to July varying between 4.5 and 1.0 ram/day. However, when soil water potential at 10 cm depth had decreased to-0.065 and -0.070 MPa in the drying phase, the evaporation from the soil decreased to 0.4 mm/day. The drainage rates were influenced by rainfall.
The highest values of capillary flux toward the surface layer, and drainage rate from the cropped soil, were 2.0 and 1.8 mm/day respectively. Rates of water uptake by roots from the soil profile, not including the 0-10 cm layer, were high when compared with drainage and upward fluxes, changing between 7.7 and 1.4 mm/day during the experimental period. A good agreement between root length densities and water uptake was found; up to 80% of all roots were in the top 50 cm of the soil and 78% of the total water uptake was extracted from the same layer. Evapotranspiration was found to decline as a cubic function of the available water content of the top 120 cm of the soil profile.