Soil hydrology

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Terms & Synonyms	Agricultural hydrology
Official WHO Definition
Other Definitions	Agricultural hydrology is the study of water balance components intervening in agricultural water management, notably in irrigation and drainage [1] .

Contents 1 Interpretations and Explanations 2 Water balance components 3 Combined balances 4 Salt balances 5 Irrigation/drainage 6 References 7 See also 8 External Resources 9 Attachments

Interpretations and Explanations

Water balance components

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 ^[2] :

1. the surface reservoir (S)

1. the root zone or unsaturated (vadose) zone (R) with mainly vertical flows
2. the aquifer (Q) with mainly horizontal flows
3. a transition zone (T) in which vertical and horizontal flows are converted

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

1. Rai - Vertically incoming water to the surface e.g.: precipitation (including snow), rainfall, sprinkler irrigation
2. Isu - Horizontally incoming surface water. This can consist of natural inundation and/or surface irrigation

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

1. Eva - Evaporation from open water on the soil surface
2. Usu - Surface runoff (natural) or surface drainage (artificial)
3. Inf - Infiltration of water through the soil surface into the root zone

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

1. Inf - Infiltration of water through the soil surface into the root zone
2. Cap - Capillary rise of water from the transition zone

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

1. Era - Actual evaporation or evapotranspiration from the root zone
2. Per - Percolation of water from the unsaturated root zone into the transition zone

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

1. Per - Percolation of water from the unsaturated root zone into the transition zone
2. Lca - Infiltration of water from river, canal or drainage systems into the tran­sition zone, often referred to as deep seepage losses
3. Ugw - Vertically upward seepage of water from the aquifer into the saturated transition zone

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

1. Cap - Capillary rise of water into the root zone
2. Dtr - Artificial horizontal subsurface drainage
3. Dgw - Vertically downward drainage of water from the saturated transition zone into the aquifer

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

1. Dgw - Vertically downward drainage of water from the saturated transition zone into the aquifer
2. Iaq - Horizontally incoming groundwater into the aquifer

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

1. Ugw - Vertically upward seepage of water from the aquifer into the saturated transition zone
2. Oaq - Horizontally outgoing groundwater from the aquifer
3. Wel - Discharge from (tube)wells placed in the aquifer

Combined balances

Water balances can be made for a combination of two bordering vertical soil zones discerned.
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, which read in general terms:

• Inflow = Outflow

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

• Rai + Isu + Cap = Eva + Era + Usu + Per

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

• Inf + Lca + Ugw = Era + Dtr + Dgw

Combination of transition zone (T) and aquifer (Q) in steady state yields the geohydrologic water balance :

• Per + Lca + Iaq = Cap + Dtr + Wel + Oaq

Combining the uppermost three water balances in steady state gives the agronomic water balance :

• Rai + Isu + Lca + Ugw = Eva + Era + Usu + Dtr + Dgw

Combining all four water balances in steady state gives the overall water balance :

• Rai + Isu + Lca + Iaq = Eva + Era + Usu + Dtr + Wel + Oaq

Salt balances

Agricultural water balances are also used in the salt balances of irrigated lands.
Further, the salt and water balances are used in agro-hydro-salinity-drainage models like SaltMod ^[3] .
Equally, they are used in groundwater salinity models like SahysMod which is a spatial variation of SaltMod using a polygonal network ^[4] .

Irrigation/drainage

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

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

Likewise the safe yield of (tube)wells, extracting water from the aquifer without over-exploitation, can be determined using the geo-hydrological 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 geo-hydrologic 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 geo-hydrologic 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 ^{[5]   [6]} .

References

1. ↑ N.A. de Ridder and J. Boonstra, 1994, Analysis of Water Balances. In: H.P.Ritzema (ed.), Drainage Principles and Applications, Publication 16, ILRI, p.601-634. International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands. ISBN 90 70754 3 39
2. ↑ R.J. Oosterbaan, 2002. Drainage and Hydrology/Salinity: Water and salt balances. On the web [1]
3. ↑ SaltMod manual, on the web : [2] . Download the model from: [3] .
4. ↑ SahysMod manual, on the web : [4] . Download the model from: [5] .
5. ↑ R.J.Oosterbaan, 1997, The energy balance of groundwater flow applied to subsurface drainage in anisotropic soils by pipes or ditches with entrance resistance. On the web : [6]
6. ↑ R.J.Oosterbaan, 2002, Subsurface drainage by (tube)wells, 9 pp. On the web : [7]

See also

Watertable control

External Resources

• Software for agro-hydro-salinity models (SaltMod, SahysMod) and calculations on horizontal and vertical subsurface drainage systems can be freely downloaded from : [8] .

Attachments