Water

Hydrological cycle

Apart from drinking water for animals, which is quantitatively minor but of great importance in arid areas, the water used in agriculture is for crops. This water is taken from watercourses, lakes, wells, etc. To avoid drought, water sources must be replenished. This is done through the hydrological cycle.

Three phases in the hydrological cycle that are of interest in agriculture:
• infiltration, the passage of water into the soil surface.
• evaporation, the process by which water is returned to the air from a liquid to a gaseous state, and the source of potential rainfall since water vapor that evaporates into the atmosphere eventually forms clouds.
• transpiration, the evaporation of water directly from plants.

Infiltration

Infiltration is particularly important in soil conservation; if the infiltration rate is high, less water passes over the soil surface, hence reducing erosion, and more water is made available to plants.

Factors affecting infiltration rate:

Antecedent moisture conditions refers to the fact that water in the soil generally reduces or limits the infiltration rate. The reduction is due in large part to the fact the water causes some colloids (very small particles in the soil) to swell and thereby reduces both the pore space and the rate of water movement. In a completely saturated soil underlain with an impervious layer, infiltration will be zero (i.e. sopping wet.)

Physical characteristics of soil such as the size of particles, the degree of aggregation between them and the arrangement of particles and aggregates; the larger the pore size and the greater the continuity of the pores that can be maintained, the greater is the resulting infiltration rate.

Land uses and crop cover are good for infiltration as it is important to maintain permanent channels, especially at the soil surface. The usual rapid reduction in the rate of intake of water through the surface is accompanied by the formation of a compact layer on the surface. This layer is the result of severe breakdown of soil structure caused in part by the beating action of raindrops and in part by an assorted action of the water flowing over the surface, fitting the fine particles around the large ones to form a relatively impervious seal. This surface sealing effect can be eliminated by protecting soil by vegetation, mulch, straw, etc. In general, vegetative cover and surface condition have more influence on infiltration rates than do the soil type and texture.

Rainfall intensity: the more intense the rate of rainfall, the larger the beating effect of raindrops. At the beginning of a rainfall, the rate of infiltration is good but it decreases rapidly, especially when the rainfall is of high intensity (soil gets saturated faster and sealing crust forms faster).

Evaporation and transpiration

For convenience, evaporation and transpiration are combined into what is called "evapo-transpiration" since it is impossible to distinguish if water vapor comes from plant transpiration or from water surface evaporation.

Plants need vast amounts of water, mainly because transpiration of water keeps them cool in the heat of the day; a hectare of cabbage transpires 3.5 million liters of water in the plants short lifetime. In addition, the nutrients taken in by the roots are transported by the water and diluted to non-toxic levels by the solution; a lack of water would lead to a reduced intake of minerals and nutrients.

Where the water supply from the soil is not limited, transpiration is regulated mostly by weather conditions, especially the amount of sun. Under these conditions, when the soil is completely covered by short, green plants - as in a grass pasture or lawn - the amount of water transpired is much the same as the evaporation from a free water surface of the same area, such as a pond or small lake. This is known as the "potential transpiration" and maximum plant growth occurs when the actual transpiration equals the potential. When the supply of water from the soil is limited, irrigation can greatly increase production.

Irrigation

Below: an irrigation pomp.

The need for irrigation is based on the amount of water being evaporated less the supply (from rainfall, etc.) of water, and it is not always easy to calculate. Since some 70% of the earth's surface is covered by water, much of it to enormous depths, it is somewhat odd that so much of the land should be so arid. The problem, of course, is that the vast majority of the water has a very high salt content.

In many parts of the world, irrigation is carried out by periodic flooding using open channels and in arid regions improper irrigation can lead to saline soils, in which very little can grow. The reason for this is that irrigation water always contains some dissolved salts and these steadily accumulate as the water evaporates. In these circumstances, excess water may have to be used to leach the unwanted salts into the drainage system.

Some crops, especially horticultural crops, are very sensitive to water supply. Seasonal periods of shortage and adverse drought climates limit crop yields. Although irrigation provides plants with adequate moisture levels, the initial investment requires a cost-benefit analysis. Since irrigation systems depend on a water supply such as a farm pond, well or stream, they must also incorporate water conservation practices to ensure sustainability amongst the farm's operations.

Drainage

Excess water also has damaging effects by preventing oxygen from reaching the roots (water-logged) and leaching nutrients from the soil. Drainage, a process which removes excess water, is also very important. In springtime, the true test of patience for a producer is to wait for a wet field to dry out before going on for soil preparation. Soil type, land topography, water table height, and seasonal precipitation all contribute to making access for machinery to the field more difficult during wet periods of the year, and, later in the season, to uneven crop yields. Soil compaction (refer to appropriate section) can occur when undertaking field work when the land is too wet and can lead to a reduction in the quality of soil structure, which in turn lowers crop yields.

Advantages of drainage:

Benefits crop yields and adds value to agricultural land; Increases efficiency of fertilizer; More rapid warming of the soil in the spring; Improves soil structure which reduces compaction; Reduces erosion since surface water run-off decreases; Controls insect pests susceptible to dryer soils.

Drainage options:

Surface drains are used as a complement or alternative to underground drainage for fine textured soils (clay, clay loam); Subsurface or underground drains collect excess water from the soil profile as the water is drawn down by gravity. Installed along the bank of watercourses and drainage ditches, they reduce ground water seepage and therefore streambank deterioration; Drop pipe inlets are large diameter pipes conveying water or intercepting surface water to a stabilized underground outlet; Rock chute spillways, accompanied by a filter cloth, accommodates anticipated overflows; Grassed waterways are protective vegetation forming a safe channel for surface water transport; Terraces are artificial earthen bergs shortening the slope length to release surface water in a more controlled fashion to a stable outlet through tile drains or grassed waterways.

Water collection structures

Surface water drained from a field and entering a watercourse should not contribute to erosion, nor damage the waterway bed. The main concern with water collection structures is preventing sediment from being transported into the watercourse, with all the negative effects attached.

Different preventative measures can be taken, such as altering the line of collection, reinforcing ditches and channels using rocks, establishment of deep-rooted vegetation and the use of special drainage equipment