Question

1. As part of the development of the one district, one factory programme, a catchment of size 100 km2 has been marked for vegetation. In its original condition, the average annual total runoff from the catchment is 1.1 m3/s. The average annual rainfall is 800 mm/a. In an average year, 50% of the rainfall infiltrates and 12.5% of the rainfall reaches the groundwater. Tests have turned out that the average annual evapotranspiration from the unsaturated zone (being the sum of the transpiration and the bare soil evaporation) amounts to 340 mm/a. In all water balance computations over the year, one may assume that the storage effects are small (dS/dt = +0).
a. How much water, in mm/a, reaches the root zone through capillary rise in an average year?
b. How much water, in cm/a, seeps out from the groundwater to the surface water in an average year?
c. How much water, in l/a, evaporates directly from interception in an average year?
d. How much, in m/a, is the total evapotranspiration in the catchment in an average year?
A well field is planned to withdraw 0.16 m3/s from the catchment for drinking water consumption elsewhere. As a result, the groundwater level is expected to go down and capillary rise into the root zone will no longer be possible. The percolation, however is expected to remain the same.
e. What will be the effect of the withdrawal on the different components of the hydrological cycle: the groundwater seepage, the total runoff, the evapotranspiration from the unsaturated zone and the total evapotranspiration? Please quantify in mm/a.

Answer 1