흥신소 Infiltration is a process that occurs when water on the ground surface enters the soil. This water may be held in depression storage, flow over the ground to a stream (surface runoff), or percolate down into the groundwater.
The type, amount, and duration of precipitation has a direct impact on infiltration rates. However, many other factors also play a role.
Water Transfer
Everywhere in the world, a portion of all precipitation soaks into soil and rock layers. This water is stored underground and serves all life’s needs. This water, through evaporation, transpiration and infiltration becomes groundwater. In addition, it seeps down into streams and is the source of “base flow.”
The process of infiltration is a complex one and is related to issues such as irrigation, contaminant transport, groundwater recharge, ecosystem viability and surface-subsurface water interaction. Quantitatively understanding this process is crucial to relating surface and subsurface processes in the hydrologic cycle.
Infiltration is a type of flow through soils that involves gravity, capillary forces, adsorption and osmosis. Infiltration is the downward movement/flow of water in soils, but it may also occur in lateral and upward directions as well. Infiltration is influenced by the distribution and rate of water supply, antecedent moisture content, soil characteristics and hydraulic properties.
The most 흥신소 significant impact on infiltration is the amount, intensity and duration of rainfall. Typically, the longer the precipitation event continues the more infiltration will occur until field capacity is reached. After this point, runoff will usually occur unless there is some physical barrier to prevent it. Infiltration measurements can be used to estimate relevant soil hydraulic parameters through a series of equations, such as Green-Ampt’s infiltration model. This model uses multiple variables including soil suction head, porosity, hydraulic conductivity and time.
Soil Hydraulic Properties
Soil hydraulic properties affect the fraction of rainfall that infiltrates versus runs off and control rates of water uptake by plants and evaporation. They also control the distribution and size of soil pores, with larger pores having a disproportionate influence on water fluxes (2).
Soils with varying textural classes, structure and landscape positions have different hydraulic properties. For example, a well-structured clayey soil with large pores will have higher saturated hydraulic conductivity than a poorly structured sandy soil with smaller pores. These differences are often seen across landscape positions and land uses (3).
In the lab, we use instrumentation to measure water content and pressure head in soils to create hydraulic conductivity curves. These curves represent a soil’s ability to move water through it at various levels of saturation/unsaturation (water potential). We call the saturated value “field saturated hydraulic conductivity” or Kfs.
These curves can be used to determine the hydraulic properties of a given soil based on its textural class or on its ability to achieve and maintain a specific water potential. However, field techniques are much more realistic and provide a better representation of the soil’s actual hydraulic properties.
One such technique involves using a series of ring infiltrometers that are inserted into the soil up to a certain depth and then used to measure infiltration rates and pressure heads. The data is then analyzed to find the relationships between the infiltration rate, water content and pressure head that are the basis of a soil’s hydraulic characteristics.
Soil Moisture Content
The amount of water held in soil particles (pore space) is the soil moisture content. It is directly related to the hydraulic properties of the soil and plays a vital role in groundwater recharge, agriculture and soil chemistry.
When it rains, some of the water passes over the ground to a stream (surface runoff), some evaporates and some infiltrates. The infiltrated water replenishes the soil moisture deficiency if any, and percolates deeper to eventually become groundwater. Soil moisture depletion also occurs during droughts through evaporation and plant transpiration.
Infiltration depends on a variety of factors, including the availability of water, the soil type and texture, its antecedent moisture content, the size and shape of the wetting front and the surface tension of the soil. The infiltration capacity of a soil decreases rapidly when the soil has been wet for several hours, because the clays within the soil swell and clog up the pore spaces.
Measuring the soil moisture is a complex process and requires laboratory equipment such as an oven or lysimeter, and requires the calibration of measuring instruments. However, there is a growing interest in the use of satellite technology to monitor changes in soil moisture in large areas over time. This information is essential to agricultural management and the prediction of floods, storms, and droughts.
Soil Composition
The soil composition is a mix of ingredients that vary from place to place. Each state has its own “state soil” recipe defined by its geography, climate, flora and fauna. These differences affect the characteristics of crop production and water infiltration rates.
The main components of soil are abiotic (rocks) and biotic (organic and inorganic matter). The next basic component is air which makes up about 2% to 50% of the volume of the soil. Air is important for oxygen supply to roots and for belowground microbial activities like nitrogen fixation.
Organic matter is another major constituent of soils at concentrations between 1% to 5%. It influences the infiltration by forming stable soil aggregates (also known as crumbs). Highly aggregated soil has greater pore space and is less prone to blocking by surface crusts. It also provides a habitat for earthworms which facilitate the development of continuous pores that link the surface to subsurface soil layers.
Soil structure and profile characteristics are greatly influenced by land management practices such as cropping and tillage. When soils are tilled or subjected to equipment traffic they tend to lose their natural structure and become compacted with decreased porosity. As a result, water infiltration into such soils is much slower than in undisturbed land.