Agricultural hydrology

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 surface reservoir (S) the root zone or unsaturated (vadose zone) (R) with mainly vertical flows the aquifer (Q) with mainly horizontal flows a transition zone (T) in which vertical and horizontal flows are converted The general water balance reads: inflow = outflow + change of storage 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: Rai – Vertically incoming water to the surface e.g.: precipitation (including snow), rainfall, sprinkler irrigation Isu – Horizontally incoming surface water. This can consist of natural inundation or surface irrigation The outgoing water balance components from the surface reservoir (S) are: Eva – Evaporation from open water on the soil surface (see Penman equation) Osu – Surface runoff (natural) or surface drainage (artificial) Inf – Infiltration of water through the soil surface into the root zone The surface water balance reads: Rai + Isu = Eva + Inf + Osu + Ws, where Ws is the change of water storage on top of the soil surface {| class="wikitable" | bgcolor=#fafad2 | Example of a surface water balance |- | bgcolor=#faf0e6 | An example is given of surface runoff according to the Curve number method. The applicable equation is: Osu = (Rai – Ws)2 / (Pp – Ws + Rm) 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.