Hydrology (agriculture)

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

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:
# 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.

Surface water balance

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

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Root zone water balance

The incoming water balance components into the root zone (''R'') are:
#Inf – Infiltration of water through the soil surface into the root zone
#Cap – Capillary rise of water from the transition zone
The outgoing water balance components from the surface reservoir (''R'') are:
#Era – Actual evaporation or evapotranspiration from the root zone
#Per – Percolation of water from the unsaturated root zone into the transition zone
The root zone water balance reads:
* Inf + Cap = Era + Per + Wr, where Wr is the change of water storage in the root zone

Transition zone water balance

The incoming water balance components into the transition zone (''T'') are:
#Per – Percolation of water from the unsaturated root zone into the transition zone
#Lca – Infiltration of water from river, canal or drainage systems into the transition zone, often referred to as deep seepage losses
#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:
#Cap – Capillary rise of water into the root zone
#Dtr – Artificial horizontal subsurface drainage, see also Drainage system (agriculture)
#Dgw – Vertically downward drainage of water from the saturated transition zone into the aquifer
The water balance of the transition zone reads:
*Per + Lca + Ugw = Cap + Dtr + Dgw + Wt, where Wt is the change of water storage in the transition zone noticeable as a change of the level of the water table.

Aquifer water balance

The incoming water balance components into the aquifer (''Q'') are:
#Dgw – Vertically downward drainage of water from the saturated transition zone into the aquifer
#Iaq – Horizontally incoming groundwater into the aquifer
The outgoing water balance components from the aquifer (''Q'') are:
#Ugw – Vertically upward seepage of water from the aquifer into the saturated transition zone
#Oaq – Horizontally outgoing groundwater from the aquifer
#Wel – Discharge from (tube)wells placed in the aquifer
The water balance of the aquifer reads:
*Dgw + Iaq = Ugw + Wel + Oaq + Wq
where Wq is the change of water storage in the aquifer noticeable as a change of the artesian pressure.

Specific water balances

Combined balances

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 :
* Rai + Isu + Cap = Eva + Era + Osu + Per, where the linkage factor ''Inf'' has disappeared.
Combination of root zone (''R'') and transition zone (''T'') in steady state yields the subsoil water balance :
* Inf + Lca + Ugw = Era + Dtr + Dgw, where Wr the linkage factors ''Per'' and ''Cap'' have disappeared.
Combination of transition zone (''T'') and aquifer (''Q'') in steady state yields the geohydrologic water balance :
*Per + Lca + Iaq = Cap + Dtr + Wel + Oaq, where Wr the linkage factors ''Ugw'' and ''Dgw'' have disappeared.

Combining the uppermost three water balances in steady state gives the agronomic water balance :
* Rai + Isu + Lca + Ugw = Eva + Era + Osu + Dtr + Dgw, where the linkage factors ''Inf'', ''Per'' and ''Cap'' have disappeared.
Combining all four water balances in steady state gives the overall water balance :
* Rai + Isu + Lca + Iaq = Eva + Era + Osu + Dtr + Wel + Oaq, where the linkage factors ''Inf'', ''Per'', ''Cap'', ''Ugw'' and ''Dgw'' have disappeared.

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Water table outside transition zone

When the water table is above the soil surface, the balances containing the components ''Inf'', ''Per'', ''Cap'' are not appropriate as they do not exist.
When the water table is inside the root zone, the balances containing the components ''Per'', ''Cap'' are not appropriate as they do not exist.
When the water table is below the transition zone, only the ''aquifer balance'' is appropriate.

Reduced number of zones

Under specific conditions it may be that no aquifer, transition zone or root zone is present. Water balances can be made omitting the absent zones.

Net and excess values

Vertical hydrological components along the boundary between two zones with arrows in the same direction can be combined into ''net values'' .

For example, : Npc = Per − Cap (net percolation), Ncp = Cap − Per (net capillary rise).

Horizontal hydrological components in the same zone with arrows in same direction can be combined into ''excess values'' .

For example, : Egio = Iaq − Oaq (excess groundwater inflow over outflow), Egoi = Oaq − Iaq (excess groundwater outflow over inflow).

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.

Equally, they are used in groundwater salinity models like SahysMod which is a spatial variation of SaltMod using a polygonal network.

Irrigation and drainage requirements

The ''irrigation requirement'' (Irr) can be calculated from the ''topsoil water balance'', the ''agronomic water balance'' 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'' 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'' 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'' 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:,Subsurface drainage by (tube)wells, 9 pp. ''Well spacing equations for fully or partially penetrating wells in uniform or layered aquifers with or without entrance resistance''. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On the web

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References

External links

* Website on agricultural hydrology

* Free software for calculations on agricultural hydrology

* Articles on agricultural hydrology

* Frequently asked questions about agricultural hydrology

* Case studies on agricultural hydrology
Water Footprint of Crops , Visual.ly

{{Agricultural water management

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