Soil Moisture Deficit
August, like June and July, is a dry month. Potential evapotranspiration still exceeds precipitation and the difference is a -42 mm. Up until this month there has been enough water from precipitation and what is in storage to meet the demands of potential evapotranspiration. However, August begins with only 16 mm of water in storage (ST of July). Thus we'll only be able to extract 16 mm of the 42 mm of water needed to meet the demands of potential evapotranspiration So, of the 42 mm of water we would need (P-PE) to extract from the soil. In so doing, the amount in storage (ST) falls to zero and the soil is dried out. What happens to the remaining 26 mm of the original P-PE of 42? The unmet need for water shows up as soil moisture deficit. In other words, we have not been able to meet our need for water from both precipitation and what we can extract from storage. AE is therefore equal to 100 mm (84 mm of precipitation plus 16 mm of ΔST). So what is a farmer to do if their crops cannot obtain needed water from precipitation or soil moisture storage....they irrigate. Irrigation water usually is pumped from groundwater supplies held in aquifers deep below the surface or from nearby streams (if stream flow is sufficient to provide needed water). The amount of irrigation water required is the amount of the soil moisture deficit. |