Water fluxes across the border of a small land territory

At present about 12% of total evaporation from the Earth s surface is used by plants in the process of photosynthesis. In this process about 2,250 km3 of water participate annually with a return coefficient of 0.75. Therefore, the simplest description of transpiration is wST(t,i,j) = 0ijWs(t,i,j), where /3,7 depends on vegetation productivity. The values of / ,-7 are 0.67 for forests, 0.44 for grassland, and 0.25 for agricultural crops. However, in real situations Ws is a limiting factor in the more 

One of the models in the process of transpiration can be written as wST = Ys 2Aa + b, where Ys is the return of water a is the rate at which ground water rises (cm/hr) and b is the average daily change in ground water level (cm). 

If value WAmax(i,j) corresponds to the real critical value of the moisture content in the atmosphere over Qiy-, then wAS has been overestimated. It is assumed here that at WA(t, ij) WAmax cloud fills the whole cell Q y, which does not always correspond with reality. Moreover, the fact that a considerable amount of moisture, even exceeding the critical level, can remain in the cloud and evaporate is disregarded. Therefore, to take these special features into account, the adaptive coefficient aw 1 should be introduced. 

Let us divide precipitation into two basic types solid and liquid. This division can be made by means of the thermal principle or by seasons. The thermal principle is preferable due its flexibility in the event of sudden climate change and possible shifting of seasonality. Synoptic division of the seasons that experience different precipitation is justified in various regions of the globe. The average daily temperature at the onset of solid precipitation (snow) is below zero, ranging from —4°C to —7°C. On the border of this division the precipitation of a different type is observed. The relationship between the types of precipitation is described by the formula xT = a — bT, where xT is the share of solid precipitation T is temperature and a and b are empirical coefficients. For the Atlantic climatic zone a = 50 and b = 5. 

To parameterize the process of evaporation from land, numerous formulas are used. Here a simple dependence is assumed  

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Figure 4.11. Water fluxes across the border of a small land territory.

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