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Hydrology (agriculture)
Encyclopedia
Agricultural hydrology is the study of water balance components intervening in agricultural water management, notably 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 outgoing water balance components from the surface reservoir (S) are:
The surface water balance reads:
The outgoing water balance components from the surface reservoir (R) are:
The root zone water balance reads:
The outgoing water balance components from the transition zone (T) are:
The water balance of the transition zone reads:
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
.
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 :
Combination of transition zone (T) and aquifer (Q) in steady state yields the geohydrologic water balance :
Combining the uppermost three water balances in steady state gives the agronomic water balance :
Combining all four water balances in steady state gives the overall water balance :
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.
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).
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.
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:
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 :
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:).
Irrigation
Irrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...
and drainage
Drainage
Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...
.
Water balance components
Water balance
In hydrology, a water balance equation can be used to describe the flow of water in and out of a system. A system can be one of several hydrological domains, such as a column of soil or a drainage basin....
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 zoneVadose zoneThe vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the top of the phreatic zone i.e. the position at which the groundwater is at atmospheric pressure . Hence the vadose zone extends from the top of the ground surface to the water table...
) (R) with mainly vertical flows - the aquiferAquiferAn aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
(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
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...
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), rainRainRain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
fall, sprinkler irrigation - Isu - Horizontally incoming surface water. This can consist of natural inundation and/or surface irrigationIrrigationIrrigation may be defined as the science of artificial application of water to the land or soil. It is used to assist in the growing of agricultural crops, maintenance of landscapes, and revegetation of disturbed soils in dry areas and during periods of inadequate rainfall...
The outgoing water balance components from the surface reservoir (S) are:
- Eva - EvaporationEvaporationEvaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
from open water on the soil surface (see Penman equationPenman equationThe Penman equation describes evaporation from an open water surface, and was developed by Howard Penman in 1948. Penman's equation requires daily mean temperature, wind speed, relative humidity, and solar radiation to predict E...
) - Osu - Surface runoffSurface runoffSurface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...
(natural) or surface drainage (artificial) - Inf - InfiltrationInfiltration (hydrology)Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes...
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
| Example of a surface water balance |
| An example is given of surface runoff according to the Curve number Curve number The runoff curve number is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess... method. The applicable equation is:
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). |
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 evapotranspirationEvapotranspirationEvapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
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)Drainage system (Agriculture)An agricultural drainage system is a system by which the water level on or in the soil is controlled to enhance agricultural crop production.-Classification:Figure 1 classifies the various types of drainage systems...
- 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)wellsWater wellA water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
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
Artesian aquifer
An artesian aquifer is a confined aquifer containing groundwater under positive pressure. This causes the water level in a well to rise to a point where hydrostatic equilibrium has been reached. This type of well is called an artesian well...
.
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.
| Example of an overall water balance | ||||||||||||
| An example is given of the reuse of groundwater for irrigation by pumped wells. The total irrigation and the infiltration are:
The field irrigation efficiency (Ff < 1) is:
The value of Era is less than Inf , there is an excess of irrigation that percolates down to the subsoil (Per):
The percolation Per is pumped up again by wells for irrigation (Wel), hence:
With this equation the following table can be prepared:
It can be seen that with low irrigation efficiency the amount of water pumped by the wells (Wel) is several time greater than the amount of irrigation water brought in by the canal system (Irr). This is due to the fact that a drop of water must be recirculated on the average several times before is used by the plants. |
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 and/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
SaltMod
SaltMod is computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the watertable, and the drain discharge in irrigated agricultural lands, using different hydrologic conditions, varying water management options, including the use of...
.
Equally, they are used in groundwater salinity models
Groundwater model
Groundwater models are computer models of groundwater flow systems, and are used by hydrogeologists. Groundwater models are used to simulate and predict aquifer conditions.-Characteristics:...
like SahysMod
SahysMod
SahysMod is a computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the watertable, and the drain discharge in irrigated agricultural lands, using different hydrogeologic and aquifer conditions, varying water management options, including...
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 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
Surface irrigation
Surface irrigation is defined as the group of application techniques where water is applied and distributed over the soil surface by gravity. It is by far the most common form of irrigation throughout the world and has been practiced in many areas virtually unchanged for thousands of years.Surface...
, 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
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
, 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
Groundwater energy balance
The groundwater energy balance is the energy balance of a groundwater body in terms of incoming hydraulic energy associated with groundwater inflow into the body, energy associated with the outflow, energy conversion into heat due to friction of flow, and the resulting change of energy status and...
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
Well drainage
Well drainage means drainage of agricultural lands by wells. Agricultural land is drained by pumped wells to improve the soils by controlling water table levels and soil salinity.-Introduction:...
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
Drainage system (Agriculture)
An agricultural drainage system is a system by which the water level on or in the soil is controlled to enhance agricultural crop production.-Classification:Figure 1 classifies the various types of 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).
The quantity of water to be drained in a normal winter is:
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 Cumulative frequency analysis Cumulative frequency analysis is the applcation of estimation theory to exceedance probability . The complement, the non-exceedance probability concerns the frequency of occurrence of values of a phenomenon staying below a reference value. The phenomenon may be time or space dependent... of climatic data plays an important role in the determination of the irrigation and drainage needs in the long run. |
External links
- Website on agricultural hydrology : http://www.waterlog.info
- Free software for calculations on agricultural hydrology : http://www.waterlog.info/software.htm
- Articles on agricultural hydrology : http://www.waterlog.info/articles.htm
- Frequently asked questions about agricultural hydrology : http://www.waterlog.info/faqs.htm
- Case studies on agricultural hydrology : http://www.waterlog.info/annrep.htm