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 material, consisting of permeable or fractured rock, or of unconsolidated materials. 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 lead to the formation of 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 groundwater, which may be fresh, saline, or brackish, depending on the locality. It can also be simply explained as the depth below which the ground is saturated.
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.
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.
In hydrogeology, groundwater flow is defined as the "part of streamflow that has infiltrated the ground, entered the phreatic zone, and has been discharged into a stream channel or springs; and seepage water." It is governed by the groundwater flow equation. Groundwater is water that is found underground in cracks and spaces in the soil, sand and rocks. Where water has filled these spaces is the phreatic saturated zone. Groundwater is stored in and moves slowly through layers or zones of soil, sand and rocks: aquifers. The rate of groundwater flow depends on the permeability and the hydraulic head.
Soil salinity control refers to controlling the process and progress of soil salinity to prevent soil degradation by salination and reclamation of already salty (saline) soils. Soil reclamation is also known as soil improvement, rehabilitation, remediation, recuperation, or amelioration.
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.
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. The part above the water table is the 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.
Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. These resources can be either freshwater from natural sources, or water produced artificially from other sources, such as from reclaimed water (wastewater) or desalinated water (seawater). 97% of the water on Earth is salt water and only three percent is fresh water; slightly over two-thirds of this is frozen in glaciers and polar ice caps. The remaining unfrozen freshwater is found mainly as groundwater, with only a small fraction present above ground or in the air. Natural sources of fresh water include surface water, under river flow, groundwater and frozen water. People use water resources for agricultural, industrial and household activities.
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 demand. 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.