The water balance basically looks at the balance between inputs and outputs. You can look at the water balance at a global level (hydrological cycle), at a local level (drainage basin cycle) or even just a field. At a global level oceans tend to experience greater outputs (evaporation) than inputs (precipitation). This is because oceans are large unshaded bodies of water that have regular winds blowing saturated air on land, allowing greater evaporation. In addition oceans don't tend to suffer from the same amount of relief and convectional rainfall as land does. On land, inputs (precipitation) tends to be greater than outputs (evaporation). This is because lands suffers from larger amounts of frontal, relief and convectional rainfall, as well as much of the lands water being protected underground or in shaded areas reducing evaporation. At a global level there obviously has to be an equilibrium between inputs and outputs. The excess precipitation on land is returned to the oceans by channel flow, surface run-off and to a lesser extent groundwater flow. The excess of evaporation is returned to the land from the sea by winds blowing saturated air on land.
At a more local level, the following formula is usually used to calculate the water balance:
S = P - Q - E
S = Stores and transfers
P = Precipitation
Q = River discharge
E = Evapotranspiration
Because drainage basins are open systems, there can be an imbalance in inputs and outputs.
At even more local level the balance between precipitation and evaporation is looked at using a soil moisture budget graph. Soil moisture graphs are useful because farmers, water providers, etc. can calculate when there will be surpluses and shortages. As such they can plan to when they have to rely on other sources like aquifers and reservoirs. However, they are limited because they look at yearly averages, so don't take into account months and years of above average or below average temperatures and rainfall.
Soil moisture excess: When soil moisture and groundwater is replenished. The excess may lead to saturation and increased surface run-off Soil moisture drawdown (usage or utilisation): When precipitation reduces and soil moisture stores begin to be used. Soil moisture deficit: When there is a shortage of soil moisture stores and deeper groundwater reserves and surface reservoirs need to be used. Soil moisture recharge: When precipitation increases and soil moisture stores fill, high infiltration and little surface run-off. Sustainable yield: The maximum extraction of water that can be maintained indefinitely.
The water balance
The water balance basically looks at the balance between inputs and outputs. You can look at the water balance at a global level (hydrological cycle), at a local level (drainage basin cycle) or even just a field. At a global level oceans tend to experience greater outputs (evaporation) than inputs (precipitation). This is because oceans are large unshaded bodies of water that have regular winds blowing saturated air on land, allowing greater evaporation. In addition oceans don't tend to suffer from the same amount of relief and convectional rainfall as land does. On land, inputs (precipitation) tends to be greater than outputs (evaporation). This is because lands suffers from larger amounts of frontal, relief and convectional rainfall, as well as much of the lands water being protected underground or in shaded areas reducing evaporation. At a global level there obviously has to be an equilibrium between inputs and outputs. The excess precipitation on land is returned to the oceans by channel flow, surface run-off and to a lesser extent groundwater flow. The excess of evaporation is returned to the land from the sea by winds blowing saturated air on land.
S = P - Q - E
S = Stores and transfers
P = Precipitation
Q = River discharge
E = Evapotranspiration
Because drainage basins are open systems, there can be an imbalance in inputs and outputs.
At even more local level the balance between precipitation and evaporation is looked at using a soil moisture budget graph. Soil moisture graphs are useful because farmers, water providers, etc. can calculate when there will be surpluses and shortages. As such they can plan to when they have to rely on other sources like aquifers and reservoirs. However, they are limited because they look at yearly averages, so don't take into account months and years of above average or below average temperatures and rainfall.
Soil moisture drawdown (usage or utilisation): When precipitation reduces and soil moisture stores begin to be used.
Soil moisture deficit: When there is a shortage of soil moisture stores and deeper groundwater reserves and surface reservoirs need to be used.
Soil moisture recharge: When precipitation increases and soil moisture stores fill, high infiltration and little surface run-off.
Sustainable yield: The maximum extraction of water that can be maintained indefinitely.