Hydrologic cycle is the circulation of water between the atmosphere and the earth. It is driven by solar energy.
Components of the field hydrologic cycle
Precipitation
Infiltration
Runoff
Evaporation
Transpiration
Percolation
Capillary rise
Infiltration
A portion of precipitation is intercepted by vegetation and evaporates without reaching the soil. But once at the soil surface water may infiltrate or runoff.
If the soil surface is stable and does not form a crust, rainfall at an intensity less than or equal to the saturated hydraulic conductivity of the soil will all infiltrate the surface. Even rainfall at an intensity greater than the saturated hydraulic conductivity may all infiltrate the surface provided that the water potential gradient at the soil surface is large due to relatively dry soil (very negative matric potential) below the soil surface. With continued infiltration, however, the upper part of the profile tends to become uniformly wet so that the matric potential component of the water potential gradient goes to zero. Under these conditions, gravity is solely responsible for infiltration and infiltration decreases to a limiting value equal to the saturated hydraulic conductivity. Ponding and runoff occur once infiltration becomes less than rainfall intensity.
Infiltration rate as a function
of time for different
rainfall intensities.
Factors other than rainfall intensity and hydraulic properties of the soil affect infiltration.
Moisture conditions of soil at onset of infiltration.
Surface deterioration and crust formation. Raindrop impact may destroy surface aggregates. The detached particles clog pores and reduce hydraulic conductivity at the surface.
Vegetative cover and crop residue protect the surface soil from raindrop impact.
Slope -a greater proportion of precipitation
subject to runoff as slope increases.
Soil-Plant-Atmosphere Continuum
Via transpiration soil water passes through
plant to atmosphere down a potential gradient. Soil to root to stem to
leaves to atmosphere. Water evaporates in the intercellular air spaces
of leaves and diffuses out of stomata into the atmosphere. The potential
drop is greatest from leaves to atmosphere.
Evapotranspiration (ET)
Combined water vapor losses by evaporation (E) and transpiration (T). Meteorological and soil factors affect E and T.
Radiant energy increases ET increases
Water vapor pressure increases ET increases
Temperature increases ET increases
Wind speed increases ET increases
Soil water content increase ET increases
Relative contribution of E and T to ET varies with climate and vegetation.
Shading of soil increases E decreases
Transpiration increases E decreases
Growing season lengthens E decreases
Evaporation and Its Control
External conditions set the evaporativity and evaporation from a wet soil is sufficiently fast to meet this external demand. This happens despite decreasing unsaturated hydraulic conductivity near the soil surface to decreasing water content because the matric potential gradient (wet soil below the surface to relatively dry soil at the surface) becomes increasingly steep. However, as the soil becomes progressively drier, not only does the hydraulic conductivity continue to decreases, but the matric potential gradient passes through a maximum and begins to decrease. Therefore, the soil can no longer supply water to the surface for evaporation as rapidly as demanded by the evporativity and the rate of evaporation steadily decreases.
Evaporation rate as a function
of time for different
evaporativities.
Control of evaporative water loss is important in arid regions or during drought in humid regions. Means of control include mulches and conservation tillage practices. These reduce
Exposure to solar radiation
Surface temperature
Vapor pressure drop
Exposure to air currents
Control of Transpiration
Seeks to increase soil water available to crop. Means include
Weed control by cultivation or herbicides
Fallow cropping in semiarid areas
Liquid Losses of Water
Percolation
Runoff
Runoff occurs when rainfall rate exceeds
infiltration. Percolation or drainage is greatest when infiltration greatly
exceeds evapotranspiration.
Percolation and Groundwater
Below the soil surface is (usually) unsaturated soil. This is called the vadose zone. It extends to a saturated zone just above the water table. This saturated zone is called the capillary fringe. It is a less than atmospheric pressure and its height depends on the largest pore size.
Upward movement of water from a shallow water table replenishes soil water lost by evapotranspiration. Shallow groundwater replenished by percolation.
Groundwater environmental issues and problems include
Water mining -rate of deep groundwater
removal > rate of recharge
Saltwater intrusion due to water table
draw down in coastal areas
Contamination due to migration of dissolved
substances
The latter may also affect surface water due to lateral subsurface flow to surface bodies of water. There is a wide range of potential contaminants including agrichemical nutrients such as NO3- which may pose human and animal health problems and, along with P may lead to accelerated eutrophication of surface waters, and pesticides.
Movement of chemicals in drainage water
is enhanced by macropores which contribute to preferential water flow.
Land Drainage
Either surface or subsurface drainage is intended to improve aeration in root zone.
Surface
Expedite runoff of ponded water. Land forming to eliminate local depressional areas coupled with system of drainage ditches.
Subsurface
Design is influenced by several factors. For example, spacing is affected by the saturated hydraulic conductivity of the soil.
Soil type K (cm/d) Spacing (m)
Clay
0.2
10 - 20
Loam
2.0
20 - 40
Sandy loam
12.0
30 - 70
The drains may be temporary subsurface tunnels (mole drain) or permanent (perforated pipe).
Better aeration promotes greater rooting vigor and depth, increased activity of aerobic microorganisms responsible for release of plant nutrients from organic matter and decreased activity of anaerobic microorganisms that reduce Fe or Mn and may create toxic levels of Fe2+ or Mn2+. In addition to better aeration, drainage allows soil to warm faster and improves access for field operations.
Drainage is also important with irrigation.
A shallow water table will sustain evaporative flux at the soil surface.
This may present a salinization hazard if ground water is salty and evaporativity
is high
Irrigation
It's an ancient water management practice. But care taken to prevent build-up of salts in soil and depletion of deep ground water.
Types
Surface Inexpensive but uniformity
of distribution and use efficiency are poor.
Sprinkler Expensive but offers uniform
application and better use efficiency.
Trickle Expensive
and difficult to maintain but offers high use efficiency.