The hydrologic cycle refers to the fate of water in and on planet Earth, from the time precipitation falls on the Earth's surface until the water is returned to Earth's atmosphere. The general principle is simple, and the driving force behind it comes primarily from the Sun's solar energy.

The components and processes of the hydrologic cycle include:

Precipitation – the condensed liquid or crystalline water falling from the atmosphere, in the form of rain, hail, sleet, or snow.

Interception – the process where precipitation is caught and temporarily held in a vegetative canopy before it reaches the land surface. Some of this precipitation may be evaporated directly or evaporated directly or adsorbed by the plant, or it occurs as throughfall (i.e., water dripping off the leaves) or as stemflow (i.e., the portion that flows down the stem to the ground).

Evaporation - the process where water passes directly from its liquid or solid state to a vapor state. Evaporation can occur from vegetation, soil, or water (and ice) surfaces.

Transpiration – the process where water is extracted from soil by plants, passing up through the plant to the plant leaves and then discharged to the atmosphere through the stomata.

Evapotranspiration – the combined processes of evaporation and transpiration.

image source: http://www.ec.gc.ca/Water/images/nature/prop/hydrologic.htm

The water "budget" for a soil profile refers to the water additions, subtractions, and the amount of water stored or remaining in the soil. The hydrologic cycle component which adds water to the soil is Infiltration, and Evapotranspiration processes are components of water removal.

Within a particular volume of soil, such as the plant root zone, is the soil water, the amount of water that the soil volume can store. The available soil water holding capacity refers to the amount of water the soil can hold, that is considered available to plants for evapotranspiration purposes.

The characteristics of rainfall are the amount, the intensity, the duration, the frequency or return period, and the seasonal distribution.

The amount is of course important to the overall hydrologic cycle and replenishment of the soil water, and the amount is an accumulation or product of the intensity times the duration. For example, the amount may be the same for a high intensity short duration rainfall as it is for a low intensity long duration rainfall.

However, the intensity and duration can have a large influence on whether the rainfall infiltrates or becomes surface runoff. Higher rainfall intensity produces larger size raindrops which have more impact energy, and thus higher intensity storms can damage delicate vegetation and bare soil.  High intensity storms can literally displace soil particles, causing soil crusting or starting the soil erosion process. High intensity storms may also overwhelm the soils ability to infiltrate the rainfall at the same rate, causing infiltration-excess runoff.

1. Soil Infiltration
2. Evaporation and Transpiration
3. Leaching
4. Runoff
5. Soil Water Storage

Soil Infiltration

Infiltration - the entry of water into soil as a result of gravity and soil water tension forces.

Infiltration Rate - the rate, or quantity of water per unit of time

Infiltration Capacity - how the rate is influenced when water ponds on the surface

Cumulative Infiltration - the total quantity (depth) of water infiltrated with time

Soil surface conditions can be predominant factors affecting whether or not infiltration will even occur, and these include surface sealing, the prevalence of cracks and open pores, or the development of crusts and an impermeable layer.

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testing soil infiltration rate

Evaporation and Transpiration

 The primary factors that affect the potential evaporation and transpiration, if water is readily available from soil and plant surfaces, are:

• Solar Radiation and Temperature
• Humidity and Wind

Solar radiation and temperature are the thermal (radiation and sensible heat energy) sources that cause water to evaporate from the earth's surface. The amount of heat energy needed to cause water to pass from a liquid to a gaseous state is called the latent heat of vaporization.

For a clear sky in the Northeast on the first day of summer, the maximum incoming (extraterrestrial) solar radiation potential provides enough energy to evaporate about a half inch of water per day. However, some of this solar energy is absorbed and diffused by clouds, some is reflected from soil, water, and plant surfaces (the albedo), and some goes into heating the atmosphere and the soil.

A heavy cloud cover in the lower atmosphere can significantly affect the amount of solar radiation that reaches the earth's surface. Since the incoming extraterrestrial solar radiation energy is reflected and diffused as it passes through the atmosphere, the net radiation potential reaching the earth's surface is much less. At the earth's surface, the potential evaporation and transpiration is about half to two-thirds the maximum, or generally around one-fourth inch of water per day when summer begins.

Humidity (vapor pressure) and wind are the aerodynamic forces which influence evaporation and transpiration. Humidity affects the vapor pressure gradient of the atmosphere and wind mixes and alters the vapor pressure gradient. At 100% humidity the atmosphere is saturated with as much water vapor as it can hold, so high humidity has the effect of reducing the amount of evaporation and transpiration that occurs. If there is no wind, the water vapor is not transported away from the evaporating or transpiring surface. We feel these same effects as we desire more water on a clear, hot, low humidity, windy day compared to a cloudy, cool, high humidity, still day

Leaching

Leaching is the removal of soluble material(s) from soil or other permeable material by the passage of water through it. Leaching depends on whether or not there is a soluble material dissolved in the soil water, and whether or not this soil water is moving, such as via deep percolation. Thus, many different soil chemical, physical and biological properties and interactions can influence the degree of leaching. For example, if one adds common table salt (NaCl – sodium chloride) to the soil, it can easily dissolve into separate sodium (cation) and chloride (anion) ions. Since soil (i.e., clay) particles exhibit a negative charge, some of the sodium may bind to the soil, whereas the chloride will not. When percolation occurs, the chloride is then more readily leached (transported) downward through the soil.

Different soil types have different percolation (infiltration or hydraulic conductivity) rates. So when adding NaCl to a gravelly soil, as compared to a silt loam, more chloride will be leached through the gravel because it has a higher percolation rate. Also, more sodium will be leached through the gravelly soil, compared to the silt loam because the gravelly soil also binds less of the sodium (has a lower cation exchange capacity). Similarly, nitrate-nitrogen is more subject to leaching than ammonium-nitrogen because of its solubility and availability in the soil water, and more of each will be leached through a gravelly soil because of its higher percolation rate.

Runoff

Surface runoff or overland flow occurs when the soil cannot infiltrate water fast enough or when infiltration ceases, and there is no further capacity to store the water near the soil surface. When the rain intensity is greater than the soils infiltration rate, the excess free water accumulates at the soil surface. This infiltration-excess may quickly result in overland flow if there is no vegetation or surface depressions that help store it, and the soil surface is sloped. A vegetative cover on the soil is very important to intercepting and dispersing the raindrop impact energy of high intensity rainfall events and in slowing the movement of overland flow when it occurs, but a thick vegetation mat or sod thatch can also provide some water storage.

Surface storage or depressional storage refers to the amount of water that can be held on the soil surface before overland flow occurs, and small depressions on roughly tilled fields may hold considerable amounts of water, delaying the onset of overland flow. Infiltration-excess runoff depends on the soil's infiltration rate, and may occur anywhere in the landscape where soils with low infiltrability occur. Infiltration-excess type runoff rarely occurs on the coarser textured soils or in soils with extensive macropore development.

When the amount of rainfall is greater than the cumulative infiltration capacity of the soil, infiltration ceases and the excess free water accumulates at the soil surface as a result of saturation-excess. Soils with shallow impermeable layers or poorly drained soils where water tables are close to the surface have limited cumulative infiltration capacity, and are thus quickly vulnerable to saturation-excess runoff. Saturation-excess runoff is not sensitive to rain intensity or soil type (infiltration rate), but depends on where the saturated areas occur in the landscape.

Water that infiltrates and is in excess of the soils available water holding capacity is subject to seepage flow or groundwater runoff. In flat landscapes this excess water causes a rise in the water table, saturates the soil and this saturation-excess recharges and fills streams, or eventually leads to flooding conditions. In sloping landscapes, the excess deep percolation also raises water tables, concentrating the water to lower portions of the landscape and recharging streams. In sloping landscapes where the subsoil is comprised of layers which conduct water at different rates, interflow above the lesser conductive layers redistributes water laterally to lower parts of the landscape, saturating low lying areas and in some situations producing springs.

Soil Water Storage

The soil water storage or soil water content can be quantified on the basis of its volumetric or gravimetric water content. The volumetric water content is the volume of water per unit volume of soil, expressed as a percentage of the volume. The gravimetric water content is the mass of water per unit mass of dry (or wet) soil. The volumetric water content is equal to the gravimetric water content times the soil's bulk density (on a dry soil basis).

Factors that affect the soil water storage are:

• Total Porosity or Void Space
• Pore-size and Distribution and Connectivity
• Soil Water Pressure Potential or Energy Status of the Soil Water

The total porosity or void space ultimately establishes the upper limit of how much water can be stored in a given volume of soil. When all the pores are filled with water the soil is saturated, and cannot store any more water. The total porosity is a function of the soil's particle size, particle uniformity and packing or structure because the void space that remains between the solid particles determines the extent and distribution of pore sizes and their connectivity.

If one fills the same volume with sand and clay sized particles, the total porosity of the clay is somewhat higher, about 50-55% of the volume compared to about 35-40% for sand. The spaces between the sand particles will have larger voids, but there will be fewer of them. The total porosity of medium textured loamy soils is generally around 50% because the smaller silt and clay particles fill some of the voids between the larger sand particles. Soils with good structure will have somewhat higher total porosity than soil that has been compacted (i.e., where the soil particles are forced closer together).

The important influence of pore-size and distribution on soil water storage is in regards to how different pore sizes respond to energy forces or the soil water pressure potential. Under saturated conditions, large pores drain more easily in response to gravity potential. Also, when the soil is unsaturated, large pores are less subject to capillary (or matric potential) forces. In unsaturated soil conditions, the soil water pressure potential becomes negative (suction), and the degree to which this occurs greatly influences the soil water storage (retention) or water content in different sized pores.

The soil water characteristic (retention) curve defines the relationship between the soil water pressure potential or energy status (matric or suction potential) and the soil water content. It's important to note that soil water moves in direct response to the energy or pressure potential forces acting upon it (i.e., moving from a higher to lower energy status), and not necessarily in response to different soil moisture contents (i.e., from higher to lower soil moisture content).

The soil water characteristic curve(s) and definitions are used to establish and further refine and quantify the general availability of soil water which is often referred to as (1) gravitational water (water subject to drainage), (2) capillary water (water available to plants), and (3) hygroscopic water (water that is not available to plants). The following figure shows general soil water characteristic curves for various soil types.

Differences in soil water pressure potentials from one point to another in the soil and throughout the larger landscape determine how water will move. For water movement in soil, the water table is used as a convenient reference because below the water table the total porosity of the soil is saturated, and above the water table, the soil porosity is unsaturated (the soil water content is less than the total porosity).

The water table is defined as the upper surface of groundwater (saturated zone) or that level in the ground below the soil surface where the water is at (and in equilibrium with) atmospheric pressure. At the water table reference, the pressure potential is set equal to zero. Thus, below the water table, the pressure potential becomes positive, and above the water table the pressure potential becomes negative. This negative pressure in unsaturated soil is termed matric, tension or suction pressure potential so as not to confuse it with positive pressures.

image source: NRCCA Soil and Water Management Study Guide

Field Capacity

The field capacity is the amount of water remaining in the soil a few days after having been wetted and after free drainage has ceased. The matric potential at this soil moisture condition is around - 1/10 to – 1/3 bar. In equilibrium, this potential would be exerted on the soil capillaries at the soil surface when the water table is between 3 to about 10 feet below the soil surface, respectively. The larger pores drain first so gravity drainage, if not restricted, may only take hours, whereas in clay soils (without macropores); gravity drainage may take two to three days. The volumetric soil moisture content remaining at field capacity is about 15 to 25% for sandy soils, 35 to 45% for loam soils, and 45 to 55% for clay soils.

Permanent Wilting Point

The permanent wilting point is the water content of a soil when most plants (corn, wheat, sunflowers) growing in that soil wilt and fail to recover their turgor upon rewetting. The matric potential at this soil moisture condition is commonly estimated at -15 bar. Most agricultural plants will generally show signs of wilting long before this moisture potential or water content is reached (more typically at around -2 to -5 bars) because the rate of water movement to the roots decreases and the stomata tend to lose their turgor pressure and begin to restrict transpiration. This water is strongly retained and trapped in the smaller pores and does not readily flow. The volumetric soil moisture content at the wilting point will have dropped to around 5 to 10% for sandy soils, 10 to 15% in loam soils, and 15 to 20% in clay soils.

Available Water Capacity

The total available water (holding) capacity is the portion of water that can be absorbed by plant roots. By definition it is the amount of water available, stored, or released between field capacity and the permanent wilting point water contents. The average amount of total available water in the root zone for a loam soil is indicated by the area between the arrows in the table on page 13.

The soil types with higher total available water content are generally more conducive to high biomass productivity because they can supply adequate moisture to plants during times when rainfall does not occur. Sandy soils are more prone to drought and will quickly (within a few days) be depleted of their available water when evapotranspiration rates are high. For example, for a plant growing on fine sand with most of its roots in the top foot of soil, there is less than one inch of readily available water.

A plant transpiring at the rate of 0.25 inches per day will thus start showing stress symptoms within four days if no rainfall occurs. Shallow rooted crops have limited access to the available soil water, and so shallow rooted crops on sandy soils are particularly vulnerable to drought periods. Irrigation may be needed and is generally quite beneficial on soils with low available water capacity.

Soil Type

Total Available Water, %

Total Available Water, in/ft

coarse sand

5

0.6

fine sand

15

1.8

loamy sand

17

2.0

sandy loam

20

2.4

sandy clay loam

16

1.9

loam

32

3.8

silt loam

35

4.2

silty clay loam

20

2.4

clay loam

18

2.2

silty clay

22

2.6

clay

20

2.4

peat

50

6.0

Total Soil Water Storage Capacity

The total soil water storage capacity refers to when all the soil pores or voids are filled with water. This occurs when the soil is saturated or flooded. A peat soil usually has the highest total soil water storage capacity of around 70 to 85% by volume. Sands and gravels will have the lowest total porosity of around 30 to 40% by volume. Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range from 40 to 60%. Restricted drainage conditions can cause the soil to attain its total porosity water content, at which time free water is observed and perched water tables develop (in layered soils) or the apparent water table is found near the surface.

When the total soil water storage capacity is reached, air is pushed out of the pores or void spaces and oxygen and other gaseous diffusion in the soil is severely restricted. Most agricultural plants cannot tolerate this condition very long (usually no more than a day or two) as plant root respiration requires some oxygen diffusion to the roots. Without air-filled pores, the concentration of carbon dioxide and other gases like ethylene increase, producing toxic conditions and limiting plant growth. Root cells switch to anaerobic respiration, which is much less efficient than aerobic respiration in converting glucose molecules to ATP (adenosine triphosphate, the chemical energy within cells for metabolism and cell division).

As anaerobic (reduced) conditions develop in the soil, nitrification ceases and denitrification is enhanced. Corn plants will quickly yellow in response to this saturated soil state as nitrogen becomes limiting, and the plant tries to adjust by producing more adventitious roots. Prolonged anaerobic conditions in the soil starts to reduce manganese, iron (causing phosphorus to be more soluble), sulfur (producing hydrogen sulfide), and eventually methane gases. Hydrophytic (wetland type) plants are adapted to saturated soils because they are able to obtain oxygen through other forms of plant structure adaptations (i.e. pneumataphores, lenticels, aerenchyma). 

Drainable Porosity   

The drainable porosity is the pore volume of water that is removed (or added) when the water table is lowered (or raised) in response to gravity and in the absence of evaporation. Consider a soil that is saturated with the water table at the surface. If this soil has a subsurface drainage pipe (tile) buried several feet down and it is discharging to the atmosphere at some lower elevation, the drainable porosity water content will be released to the tile drain until the water table is lowered to the depth of the drain.    

Any nutrients or pesticides dissolved or suspended in this readily drainable pore space will also be carried along with this water, either flowing to the tile drain or continuing downward to the water table via deep percolation if no drainage restriction exists. In large pores, nutrients that might otherwise adsorb to the soil particles (ammonium or phosphate) will bypass the soil because of limited time for contact and chemical reactions to occur with the soil surface area. Soils with a wide range of different pore sizes (sandy loams) or soils with mostly small sized pores are better at filtering nutrients and pesticides as they leach through the soil profile. 

The combined aspect of low available water holding capacity and high drainable porosity for sandy soils causes these soils to have a high leaching potential. It will not take much rain or irrigation (or application of liquid manure) to replenish the available soil water and to raise the soil water content to a drainable state. Applying the proper amount (depth) of irrigation to these soils will both conserve water and enhance irrigation and nutrient use efficiency.

Soil Texture and Structure

The size and percentage of individual soil particles determines a soil's texture, but when the bulk arrangement of these particles is considered, the term structure is used. The clustering or aggregation of primary soil particles into compound particles of naturally formed peds or separate soil aggregates can greatly modify the textural influence on soil air and moisture relationships. The soil texture and structure fundamentally determines the number and sizes of soil pores, which will influence the fate and transport of air (gas) and water exchange. Various soil forming processes lead to natural structural features, and soil scientists classify these into the types shown below.

Pores exist around these structure types, and some (i.e., crumb, subangular blocky) will also contain smaller micro-pores within the aggregate or ped.  A platy structure usually impairs permeability because the horizontal plates often overlap, but a blocky structure may lead to even larger pore sizes than would be found between individual soil particles.  The blocky structure is important in clay soils, and is generally what provides the drainable porosity.

image source: NRCCA Soil and Water Management Study Guide

Soil structure can be modified naturally by soil microorganisms. The effect of worm burrows in forming macropores is perhaps best known because it can be easily visualized. However, modern tillage implements also greatly modify the soil structure within, and sometimes below (plowpan), their range of depth influence. The following figure provides an illustration of how various parameters of soil water storage may be influenced by different texture and structure aspects (From H.M. van Es).

images' source: NRCCA Soil and Water Management Study Guide

Macroporosity/Preferential Flow

Macropores refers to those soil pores through which water flows primarily in response to gravity. Macropores occur in coarse sands and gravels, soil structural cracks, or may form as the result of worm holes, other small burrowing microorganisms, decaying roots, and some tillage operations. Since water can be infiltrated quickly and flows rapidly downward in macropores, it is also termed preferential flow.

The significance of macropores and preferential flow is that nutrients and other dissolved and suspended substances can be rapidly transported down past the root zone without substantial filtration or other biochemical remediating interactions. Although the magnitude of macroporosity in soils is generally small, when only a small concentration amount of a nutrient, pesticide or other contaminant creates great risk to water quality, the environmental threat may still be significant. Macroporosity is generally beneficial to air and water exchange, soil health, and to providing more optimum conditions for plant growth, but it has also lead to water quality impacts when dissolved and suspended materials are transported to tile drain outlets or groundwater.

Permeability is the ease with which gases, liquids, or plant roots can penetrate or pass through a layer of soil or porous media. Permeability is thus both a feature of the soil pore size and distribution for allowing something to pass through it; and a feature of the gas, liquid, or plant root in its ability to diffuse, flow, or penetrate through these soil pore openings.

One analogy of intrinsic permeability is screens with different size openings. The screen with the biggest opening is more intrinsically permeable than one with the smallest opening. The opening size may have little effect on a gas passing through, may effect on how water can move through, and a major effect on what size roots might grow through.

Since soil gases primarily move through the larger (non-water filled) pores, compaction which compresses the larger pores first can substantially diminish the soils ability for gaseous exchange. Saturation of the pores also severely restricts gas permeability and the exchange of soil air with the atmosphere.

Liquids differ greatly from air with regards to flow properties. Water is much denser than air, and water viscosity changes more dramatically over normal environmental temperatures. Because of changes in density and viscosity, water at 35 degrees Fahrenheit will not infiltrate soil quite as quickly as warmer water. Liquid manure is thought to move more quickly than water through soil via gravity because it has a higher density and the suspended organics alter its viscosity.

Management practices which reduce the soil's permeability at the surface (makes soil pores smaller or causes them to be blocked or less connected) will reduce infiltrability. Practices such has heavy traffic loads or compressive tillage (plow, disk) which reduce pore size or connectivity at deeper depths may also reduce the soils overall permeability at deeper depths in the soil.

Management practices that reduce the soil's permeability often have an effect on the ease with which plant roots can penetrate the soil and grow too. Plant roots may have difficulty penetrating and thriving in soils with smaller pores, not only because of changes in air-water relationships, but also because of alterations to the soils bulk density and soil strength. Plant roots have a more difficult time penetrating dry soils, compared to moist soils, because of differences in soil strength at different soil water contents.

Seasonal soil conditions change in response to the seasonal distribution of precipitation relative to the distribution of the potential evapotranspiration. In the Northeast, the distribution of the annual precipitation is similar from month to month, but the evapotranspiration peaks in July and diminishes significantly in the colder months. During the months when the rainfall is higher than the evapotranspiration, the soil will be gaining water. When the rainfall is lower than evapotranspiration, the soil water is extracted. As the soil water content increases, there is a limit to how much it can hold the field capacity.

Although some surface runoff can occur anytime in response to large storms, most of the runoff (and tile drain discharge) begins after evapotranspiration ceases in the fall, and it peaks in late winter/early spring in response to all the additional water from snowmelt. Runoff starts to decrease in late spring in response to increasing evapotranspiration, which again begins to extract water from the soil. Runoff and leaching are minimal during the late summer months because the soil water content is usually less than field capacity, and the soil can store rains that occur at this time. In fact, during the summer months, irrigation may be needed to maintain the soil water content from dropping below critical thresholds. The following figure shows the distribution of the average water balance for Central NY.

 image source: NRCCA Soil and Water Management Study Guide

One simple method to assess soil water conditions in the field is the "Feel" method, a somewhat more specific approach to the "ball test". Soil will either feel wet, moist, or dry. When the soil is wet, it will leave water on your hand (about field capacity). When the soil feels moist, it likely will be somewhere in the plant available water content range. A soil that feels dry is usually below the critical threshold of the readily available water moisture content.  Sandy soils may rarely feel wet because of their high drainable porosity. For a more detailed description of the "Feel" method go to: http://www.wy.nrcs.usda.gov/technical/soilmoisture/guideline.html

• Water cycles through the environment as a solid, liquid, or gas through processes like precipitation, evaporation, transpiration, condensation, etc.  The additions or deletions of water due to these processes are considered the water "budget" for soil.
• The rates at which the processes occur, and thus the rates at which water may enter or leave a given soil system, are dependent on factors both controllable and uncontrollable by humans.
• Ecologists and farmers must consider the status of a soil (its structure, water content, etc) when planning practices like plowing, tilling, planting, or application of manure, fertilizer, or pesticides.