Agricultural hydrology is the study of water balance components intervening in agricultural water management, especially in irrigation and drainage. [1]
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]
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 incoming water balance components into the surface reservoir (S) are:
The outgoing water balance components from the surface reservoir (S) are:
The surface water balance reads:
Example of a surface water balance |
An example is given of surface runoff according to the Curve number method. [3] 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). |
The incoming water balance components into the root zone (R) are:
The outgoing water balance components from the surface reservoir (R) are:
The root zone water balance reads:
The incoming water balance components into the transition zone (T) are:
The outgoing water balance components from the transition zone (T) are:
The water balance of the transition zone reads:
The incoming water balance components into the aquifer (Q) are:
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.
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 :
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 :
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 it is used by the plants. |
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.
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.
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).
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.
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:
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 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:, [4] [5] ).
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 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 [6] of climatic data plays an important role in the determination of the irrigation and drainage needs in the long run. |
Hydrology is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydrologist. Hydrologists are scientists studying earth or environmental science, civil or environmental engineering, and physical geography. Using various analytical methods and scientific techniques, they collect and analyze data to help solve water related problems such as environmental preservation, natural disasters, and water management.
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials. Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteristics. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Related terms include aquitard, which is a bed of low permeability along an aquifer, and aquiclude, which is a solid, impermeable area underlying or overlying an aquifer, the pressure of which could create a confined aquifer. The classification of aquifers is as follows: Saturated versus unsaturated; aquifers versus aquitards; confined versus unconfined; isotropic versus anisotropic; porous, karst, or fractured; transboundary aquifer.
The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated.
Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust. The terms groundwater hydrology, geohydrology, and hydrogeology are often used interchangeably.
The vadose zone, also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone, 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.
The law of water balance states that the inflows to any water system or area is equal to its outflows plus change in storage during a time interval. 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 or water domains, such as a column of soil, a drainage basin, an irrigation area or a city. Water balance can also refer to the ways in which an organism maintains water in dry or hot conditions. It is often discussed in reference to plants or arthropods, which have a variety of water retention mechanisms, including a lipid waxy coating that has limited permeability.
Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs in the vadose zone below plant roots and is often expressed as a flux to the water table surface. Groundwater recharge also encompasses water moving away from the water table farther into the saturated zone. Recharge occurs both naturally and through anthropogenic processes, where rainwater and or reclaimed water is routed to the subsurface.
Subsurface flow, in hydrology, is the flow of water beneath earth's surface as part of the water cycle.
Groundwater models are computer models of groundwater flow systems, and are used by hydrologists and hydrogeologists. Groundwater models are used to simulate and predict aquifer conditions.
Soil salinity control relates to controlling the problem of soil salinity, with the aim of preventing soil degradation by salination and reclamation of already salty (saline) soils. Soil reclamation is also called soil improvement, rehabilitation, remediation, recuperation or amelioration.
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 the use of ground water for irrigation, and several crop rotation schedules, whereby the spatial variations are accounted for through a network of polygons.
The phreatic zone, saturated zone, or zone of saturation, is the part of an aquifer, below the water table, in which relatively all pores and fractures are saturated with water. Above the water table is the unsaturated or vadose zone.
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 (hydrology) in irrigated agricultural lands, using different (geo)hydrologic conditions, varying water management options, including the use of ground water for irrigation, and several cropping rotation schedules. The water management options include irrigation, drainage, and the use of subsurface drainage water from pipe drains, ditches or wells for irrigation.
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 groundwater level.
FEHM is a groundwater model that has been developed in the Earth and Environmental Sciences Division at Los Alamos National Laboratory over the past 30 years. The executable is available free at the FEHM Website. The capabilities of the code have expanded over the years to include multiphase flow of heat and mass with air, water, and CO2, methane hydrate, plus multi-component reactive chemistry and both thermal and mechanical stress. Applications of this code include simulations of: flow and transport in basin scale groundwater systems , migration of environmental isotopes in the vadose zone, geologic carbon sequestration, oil shale extraction, geothermal energy, migration of both nuclear and chemical contaminants, methane hydrate formation, seafloor hydrothermal circulation, and formation of karst. The simulator has been used to generate results for more than 100 peer reviewed publications which can be found at FEHM Publications.
The following outline is provided as an overview of and topical guide to hydrology:
Catchment hydrology is the study of hydrology in drainage basins. Catchments are areas of land where runoff collects to a specific zone. This movement is caused by water moving from areas of high energy to low energy due to the influence of gravity. Catchments often do not last for long periods of time as the water evaporates, drains into the soil, or is consumed by animals.
DPHM-RS is a semi-distributed hydrologic model developed at University of Alberta, Canada.
Groundwater banking is a water management mechanism designed to increase water supply reliability. Groundwater can be created by using dewatered aquifer space to store water during the years when there is abundant rainfall. It can then be pumped and used during years that do not have a surplus of water. People can manage the use of groundwater to benefit society through the purchasing and selling of these groundwater rights. The surface water should be used first, and then the groundwater will be used when there is not enough surface water to meet demands. The groundwater will reduce the risk of relying on surface water and will maximize expected income. There are regulatory storage-type aquifer recovery and storage systems which when water is injected into it gives the right to withdraw the water later on. Groundwater banking has been implemented into semi-arid and arid southwestern United States because this is where there is the most need for extra water. The overall goal is to transfer water from low-value to high-value uses by bringing buyers and sellers together.
Coastal Hydrogeology is a branch of Hydrogeology that focuses on the movement and the chemical properties of groundwater in coastal areas. Coastal Hydrogeology studies the interaction between fresh groundwater and seawater, including seawater intrusion, sea level induced groundwater level fluctuation, submarine groundwater discharge, human activities and groundwater management in coastal areas.