Studies in the water relations of Pinus sylvestris in plantation conditions. III. A comparison of soil water changes and estimates of total evaporation on four afforested sites and one grass-covered site.
(S. & F. XXVIII ). Soil-moisture changes were followed for three years; the plantations varied in age from 20 to 32 years and were on soils differing in texture and water-table depth. From the results, total evaporation during the period of soil-water deficit was calculated for each site in each year. Available water in the rooting zone was defined as that held between 0.2 and 10 atm and was estimated from suction measurements. On the forested sites roots penetrated to about 2 m and the depth of available water in the rooting zone varied from 17 to 37 cm according to soil texture. The grass roots penetrated to only 60 cm depth and depth of available water in this region was only 3.7 cm. In the forested sites potential evaporation rate was about 1.2 times that of open water (Eo). On the only site with an evaporation rate below this, results were consistent with the assumption that the potential rate was maintained until most of the available water was used and that evaporation then continued at a tenth of the potential rate. On the grass-covered site, evaporation during the months of soil-water deficit was about 0.5Eo and this was consistent with evaporation at the potential rates of 0.8Eo in May-August and 0.7Eo in September, maintained until the available water was exhausted, and then reduced to a tenth.