Water in rivers, lakes, oceans, and similar sources is referred to as surface water. It is naturally replenished by precipitation and naturally lost through discharge to evaporation and sub-surface seepage into groundwater.
Groundwater is water that exists in the pore spaces and fractures in rocks and sediments beneath the earth’s surface.
It originates as rainfall, then moves through the soil and rock into the groundwater system, where it eventually makes its way back to surface streams, lakes, or oceans.
Groundwater occurs in the sub-surface in two broad zones: the unsaturated zone and the saturated zone. The unsaturated zone, also known as the vadose zone, consists of soil pores that are filled to a varying degree with air and water. The zone of saturation consists of water-filled pores that are assumed to be at hydrostatic pressure.
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Surface Water and Its Role in Agricultural Water Supply
Water in rivers, lakes, oceans, and similar sources is referred to as surface water; it is naturally replenished by precipitation and naturally lost through discharge to evaporation and sub-surface seepage into groundwater.
Water beneath the land surface occurs in two principal zones, the unsaturated zone and the saturated zone.
In the unsaturated zone, the voids that is, the spaces between grains of gravel, sand, silt, clay, and cracks within rocks contain both air and water. In contrast to the unsaturated zone, the voids in the saturated zone are completely filled with water.
Water in the saturated zone is referred to as groundwater. The upper surface of the saturated zone is referred to as the water table.
Water Bodies and Their Agricultural Significance
A water body refers to any stream, river, pond, lake, estuary, or ocean. Water bodies can be flowing (lotic) systems (streams and rivers) or non-flowing (lentic) systems (ponds and lakes).
The flow of water in these systems, particularly in rivers and streams, is greatly affected by the natural features of the watershed, including the topography, slope, soils, and vegetation.
Surface water is exposed to many different contaminants, such as animal wastes, pesticides, insecticides, industrial wastes, algae, and many other organic materials.
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Groundwater and Its Relevance to Agricultural Use
Groundwater refers to water that completely fills pore spaces within the zone of saturation beneath the Earth’s surface. All geologic materials are composed of solids (i.e., actual grains, sediment, or rock matrix) and pore space (i.e., voids).
Most groundwater comes from precipitation, which gradually percolates into the earth. Typically, 10–20% of precipitation eventually enters aquifers.
The amount of available pore space and the interconnectivity of pores govern the storage and transmission of groundwater.
Groundwater, like runoff, flows from a higher to a lower elevation; however, geologic structure and formations also control its flow.
Areas in which the pore spaces are completely filled with water are called the saturated zone. Above the saturated zone is an area in which the pore spaces are filled with air and water, called the unsaturated or vadose zone.
The distinction between saturated and unsaturated sub-surface zones is based on the location of the water table, which is found at the top of the saturated zone (Figure 3.1), where the pore water pressure is equal to atmospheric pressure.
Aquifers and Their Agricultural Importance
The water-bearing soil or rock that is capable of yielding usable amounts of water is called an aquifer. There are two major types of aquifers: confined and unconfined.
1. Confined Aquifers in Agricultural Water Management
Confined aquifers (also known as artesian or pressure aquifers) exist where the groundwater system is between layers of clay, dense rock, or other materials with very low permeability.
Water in confined aquifers may be very old, even millions of years old. This water is under more pressure than water in unconfined aquifers. Thus, when tapped by a well, water is forced up, sometimes above the soil surface. This is how a flowing artesian well is formed.
2. Unconfined Aquifers and Their Contribution to Crop Irrigation
More common than confined aquifers, unconfined aquifers have a permeable deposit that leads into the aquifer. Water may have arrived by percolating through the land surface.
This is why water in an unconfined aquifer is often very young in geologic time. The top layer of the aquifer is also the water table. Thus, it is affected by atmospheric pressure and changing hydrologic conditions.
Agricultural Importance of Underground Water
Groundwater contributes to the generation and regulation of stream flow in watersheds, and the sustainability of many wetlands, ponds, and lakes.
Nearly 95% of rural residents rely on groundwater for their drinking supply.
About half of irrigated cropland uses groundwater.
Approximately one-third of industrial water needs are fulfilled by using groundwater.
About 40% of river flow nationwide (on average) depends on groundwater.
Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean. Groundwater is water that exists in the pore spaces and fractures in rocks and sediments beneath the earth’s surface.
Both sources are essential in agriculture, contributing to irrigation, livestock, aquaculture, and general rural livelihoods.
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