2 - water balance graphs

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Rhys Wall 1 Key Word Definition A – Soil Moisture Excess Precipitation exceeds PEVT, so water will be gained. Because antecedent moisture is high already, the soil cannot take in any more water, so the water table rises and flooding becomes most likely during this time. B – Soil Moisture Utilisation PEVT now exceeds precipitation, so water will be lost. However, vegetation and rivers will still be able to utilise the water in the water table since it was so high in the prior months. Water will flow from the soil (throughflow) and rock (groundwater flow) to the rivers. C – Soil Moisture Deficit PEVT still exceeds precipitation, so water is still being lost. This stage may not always happen as the water table may have been high enough to keep rivers at their baseflow and avoid a drought. D – Soil Moisture Recharge Precipitation begins to exceed PEVT. The water gained is used to refill the pores in the soil and rock, recharging the water table. Flooding during this time is unlikely. Water Balance Graphs A water balance graph is used to plot the fluctuation of inputs (precipitation) and outputs (evapotranspiration) and can be interpreted to analyse soil moisture. The water balance budget formula can be used to express the inputs and outputs of the drainage basin system. P = Q + E + / - S P – Precipitation E - Evapotranspiration Q – Runoff S – Change In Stores Precipitation Potential Evapotranspiration (PEVT) mm Months C D A B

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Page 1: 2 - Water Balance Graphs

Rhys Wall

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Key Word Definition A – Soil Moisture Excess Precipitation exceeds PEVT, so water will be gained. Because antecedent moisture is high already, the soil

cannot take in any more water, so the water table rises and flooding becomes most likely during this time.

B – Soil Moisture Utilisation PEVT now exceeds precipitation, so water will be lost. However, vegetation and rivers will still be able to utilise the water in the water table since it was so high in the prior months. Water will flow from the soil (throughflow) and rock (groundwater flow) to the rivers.

C – Soil Moisture Deficit PEVT still exceeds precipitation, so water is still being lost. This stage may not always happen as the water table may have been high enough to keep rivers at their baseflow and avoid a drought.

D – Soil Moisture Recharge Precipitation begins to exceed PEVT. The water gained is used to refill the pores in the soil and rock, recharging the water table. Flooding during this time is unlikely.

Water Balance Graphs

A water balance graph is used to plot the fluctuation of inputs (precipitation) and outputs (evapotranspiration) and can be interpreted to analyse soil moisture.

The water balance budget formula can be used to express the inputs and outputs of the drainage basin system.

P = Q + E + / - S

P – Precipitation E - Evapotranspiration

Q – Runoff S – Change In Stores

Precipitation Potential Evapotranspiration (PEVT)

mm

Months

C

D A

B