a drainage basin bordering the sea has an area of 7500 km2 . the average precipitation for this drainage basin is 900 mm year-1 . the average surface water flow at the outlet of the drainage basin equals 22.5 × 108 m3 year-1 . the average groundwater flow to the sea is 100 mm year-1 . the averages are determined for 30 hydrological years.
a. draw up the water balance.
b. determine the average actual evaporation in mm year-1 and m3 year-1