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Hydrology (agriculture)


Agricultural hydrology is the study of water balance components intervening in agricultural water management, especially in irrigation and drainage.

The water balance components can be grouped into components corresponding to zones in a vertical cross-section in the soil forming reservoirs with inflow, outflow and storage of water:

The general water balance reads:

and it is applicable to each of the reservoirs or a combination thereof.

In the following balances it is assumed that the water table is inside the transition zone.

The incoming water balance components into the surface reservoir (S) are:

The outgoing water balance components from the surface reservoir (S) are:

The surface water balance reads:

where Rm is the maximum retention of the area for which the method is used

Normally one finds that Ws = 0.2 Rm and the value of Rm depends on the soil characteristics. The Curve Number method provides tables for these relations.

The method yields cumulative runoff values. To obtain runoff intensity values or runoff velocity (volume per unit of time) the cumulative duration is to be divided into sequential time steps (for example in hours).

The incoming water balance components into the root zone (R) are:

The outgoing water balance components from the surface reservoir (R) are:

The root zone water balance reads:

The incoming water balance components into the transition zone (T) are:

The outgoing water balance components from the transition zone (T) are:

The water balance of the transition zone reads:

The incoming water balance components into the aquifer (Q) are:

The outgoing water balance components from the aquifer (Q) are:

The water balance of the aquifer reads:

where Wq is the change of water storage in the aquifer noticeable as a change of the artesian pressure.

Water balances can be made for a combination of two bordering vertical soil zones discerned, whereby the components constituting the inflow and outflow from one zone to the other will disappear.
In long term water balances (month, season, year), the storage terms are often negligible small. Omitting these leads to steady state or equilibrium water balances.

Combination of surface reservoir (S)and root zone (R) in steady state yields the topsoil water balance :

Combination of root zone (R) and transition zone (T) in steady state yields the subsoil water balance :


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