P=E+Q+deltaS
P= Precipitation
E=Evapotranspiration
Q=Run off.
deltaS=Change in storage.
Basically, water balance is just an expression of the idea of conservation of water: If there is water precipitated, that amount of water has to either evaporate back or run off to somewhere else.
2. What we do in agriculture and how that affects water balance:
Assume that the considered land was previously a forest, what we do is basically to chop down all the trees and vegetate the land with crops. To determine how that affects various factors in the water balance, let's make further assumption that the previous concentrated forest is replaced by corn. Based on that assumption, evapotranspiration will obviously drops since the ability to intercept and hold water of the concentrated forest is much better than the corn field. Hence E drops.
Through agricultural activity, the surface soil may be soften and made more spongy. However, as the vegetation layer is much thinner than before, this surface soil is easily washed off. The root of the crop plants is also unable to penetrate deep down into the soil, hence the effect of plants on deeper soil's capacity is eliminated. Hence, deltaS drops.
Since E and deltaS drop, Q increases (Assuming that the amount of precipitation is constant)
3. Changes in Hydrograph.
As surface run off increases, the peak of the hydrograph is expected to be higher (more overlan, the lag time is expected to be shorter and the gradient of the rising limb is steeper.
I think the graph of grazing and agriculture should be separated as we assume that the original plot of land is forest. In agriculture, vegetation is reduced. However, as grazing is a process when animals eat all the vegetation, the number of vegetation is even less. In addition, as animals walk on the soil, soil becomes more compacted and therefore increase overland flow. :D
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