Hydrology (agriculture) - Irrigation and Drainage Requirements

Irrigation and Drainage Requirements

The irrigation requirement (Irr) can be calculated from the topsoil water balance, the agronomic water balance and/or the overall water balance, as defined in the section "Combined balances", depending on the availability of data on the water balance components.
Considering surface irrigation, assuming the evaporation of surface water is negligibly small (Eva = 0), setting the actual evapotranspiration Era equal to the potential evapotranspiration (Epo) so that Era = Epo and setting the surface inflow Isu equal to Irr so that Isu = Irr, the balances give respectively:

• Irr = Epo + Osu + Per − Rai − Cap
• Irr = Epo + Osu + Dtr + Dgw − Rai − Lca − Ugw
• Irr = Epo + Osu + Dtr + Oaq − Rai − Lca − Iaq

Defining the irrigation efficiency as IEFF = Epo/Irr, i.e. the fraction of the irrigation water that is consumed by the crop, it is found respectively that :

• IEFF = 1 − (Osu + Per − Rai − Cap) / Irr
• IEFF = 1 − (Osu + Dtr + Dgw − Rai − Lca − Ugw) / Irr

• IEFF = 1 − (Osu + Dtr + Oaq − Rai − Lca − Iaq) / Irr

Likewise the safe yield of wells, extracting water from the aquifer without overexploitation, can be determined using the geohydrologic water balance and/or the overall water balance, as defined in the section "Combined balances", depending on the availability of data on the water balance components.

Similarly, the subsurface drainage requirement can be found from the drain discharge (Dtr) in the subsoil water balance, the agronomic water balance, the geohydrologic water balance and/or the overall water balance.

In the same fashion, the well drainage requirement can be found from well discharge (Wel) in the geohydrologic water balance and/or the overall water balance.

The subsurface drainage requirement and well drainage requirement play an important role in the design of agricultural drainage systems (references:, ).

Example of drainage and irrigation requirements

The drainage and irrigation requirements in The Netherlands are derived from the climatic characteristics (see figure). Climatic data in the figure (mm) Summer Apr-Aug Winter Sep-Mar Annual Precipitation P 360 360 720 Evaporation E 480 60 540 Change of storage ΔW –120 +120 0 Drainage requirement D 0 180 180 Irrigation requirement variable 0 variable The quantity of water to be drained in a normal winter is: D = P – E – ΔW According to the figure, the drainage period is from November to March (120 days) and the discharge of the drainage system is D = 180 / 120 = 1.5 mm/day corresponding to 15 m3/day per ha. During winters with more precipitation than normal, the drainage requirement increase accordingly. The irrigation requirement depends on the rooting depth of the crops, which determines their capacity to make use of the water stored in the soil after winter. Having a shallow rooting system, pastures need irrigation to an amount of about half of the storage depletion in summer. Practically, wheat does not require irrigation because it develops deeper roots while during the maturing period a dry soil is favorable. The analysis of cumulative frequency of climatic data plays an important role in the determination of the irrigation and drainage needs in the long run.