Rainfall – discharge relationship with drainage basins lesson 4
The balance between the water inputs and the water outputs Equation: P= Q + E + S Where: P= precipitation Q= total streamflow (runoff) E= Evapotranspiration S= Storage (in soil & bedrock)
used to help manage water supply and predict where there may be water shortages. also used in: irrigation, runoff assessment flood control and pollution control.
illustrated using a water balance graph plots levels of precipitation and evapotranspiration often on a monthly scale. The graph helps to highlight river-basin management challenges, which include: Periods when there is likely to be water deficiency or drought Periods when flooding is most likely to occur The best time for irrigation Long-term changes in the storage capacity of the drainage basin The need for a water transfer system
1. Soil moisture surplus: soil moisture is full for plant use and run off into streams 2 32 Soil moisture use: plants must rely on stored 4 water which gradually used up 1 53. Soil moisture deficiency: plants must have adaptation for long periods or land must be irrigated4. Soil moisture recharge: the soil water stored start to be recharged5. Field capacity: the soil water stores are now full
Annual precipitation always exceeds evapotranspiration Precipitation (Input) exceeds evt loss (output) = positive water balance soil moisture surplus However, 1974 &1975 as well as 1995 & 1996 – long dry summer Evapotranspiration exceeds precipitation temporary negative water balance During winter, ppt > evt = soil moisture surplus considerable river run off & rise in river levels In summer, evt > ppt (plants & humans utilise the water from the soil storage depletion and fall in river levels By Autumn, ppt> evt = first water surplus is to recharge the until the soil reach field capacity