The Water Balance Notes

THE WATER BALANCE: What?

The relationship between precipitation, evapotranspiration evapotranspiration and storage (in the  form of soil moisture and groundwater) can be expressed as the water balance  equation:  Precipitation (P) = streamflow (Q) + evapotranspiration evapotranspiration (E) + change in storage  (S)  * (Balance between inputs (precipitation) and outputs (streamflow/runoff, evapotranspiration evapotranspiration and storage)  Uses/importance: 1. It is used used by Hydrol Hydrologis ogists ts to plan plan and manage manage water water supply supply withi withinn a drainage basin, for example; when there is water shortages hosepipe bans are implemented to preserve stocks. 2. For irrigat irrigation, ion, pollutio pollutionn control control and control control floodin flooding. g. A water budget graph is a useful way to look at the water balance of a location over a year. Precipitation and evapotranspiration rates are plotted on to a single graph. 1. Actual evapotranspiration (AET): the loss of moisture to the atmosphere by the processes of evapotration and transpiration which actually t akes place.

Important terms: 2. Potential evapotranspiration (PET): the maximum amount amount of evapotranspiration which occur if an adequate supply of water were continuously available. (In Desert PET >AET amount of water is limited) 3. Soil moisture surplus: occurs when the soil water store is full and thus there is surplus of water for plants, runoff and groundwater recharge. (PP>PET) 4. Soil moisture utilisation: Plants (and people) use moisture stored in the soil, leaving it depleted. (AET >PP) 5. Soil moisture deficiency: Equivalent to the extra water which would be needed to maintain maximum plant growth. There There is little or no water wa ter available for plant growth (irrigation could make good this deficit) (PET > AET) 6. Soil moisture recharge: The soil water store starts to fill again after a period of  deficiency) 7. Field capacity: The moisture a freely drained soil can hold after all free or gravity water has drained away. Such moisture is held by tension around soil particles, mainl as ca illar water.

WATER BUDGET GRAPH (Soil moisture budget graph):

Ref: http://www.heinemann.co.uk/s http://www.hei nemann.co.uk/shared/Resources hared/Resources/NonSecure/00 /NonSecure/00000001/AS_Lev 000001/AS_Lev el_Geog_Edexcel.pdf *Draw the diagram of soil moisture budget graph/water budget graph)

*WATER BUDGET GRAPH: 1. What is a water budget? A water budget reflects the relationship between input and output of water through a region. The water balance graph shows precipitation and potential evapotranspiration both as line graphs. Thus we have a direct comparison of supply of water and the natural demand for water. It is possible to identify the periods when there is plent y of precipitation and when there is not enough. 2. Definition of terms: The following terms will be used in the questions that follow: •

•

•

•

Potential Evapotranspiration (PE): (PE): All the water that could enter the air from plants and evaporation if present. Precipitation (P): (P): All moisture from the atmosphere, rain, snow, hail and sleet. Surplus: Surplus: Water above what is lost naturally from the soil (when P is greater  than PE) Deficit: Deficit: Water that would be lost above what is in the soil if i t were present (when P is less than PE)