The Hydrologic Cycle and ET

ET is a major component ofthe hydrologic cycle. A major proportion of the total precipitation falling on the land surface is returned to the atmosphere by ET. As a global average, 57% of the annual precipitation falling over the land is returned to the atmosphere by ET. ET amounts to about 70% of the annual precipitation of the United States, and more than 90% of the precipitation in the arid and semiarid areas of the western United States. Different components of a typical hydrologic cycle are illustrated in Figure 1. The hydrologic cycle can be defined as the pathways ofwater as it moves in its various phases through the atmosphere, to the Earth, over and through the land, to the ocean, and back to the atmosphere. During this cycle, which has no beginning or end, water molecules may assume various states, returning to a hydrologic pathway as new chemical compounds that are mixed with various solid and liquid substances. In the cycle, water evaporates from the oceans, ponds, rivers, and various land surface to become part of the atmosphere; water vapor is transported and lifted in the atmosphere until it condenses and precipitates on the land or oceans. Precipitated water may be intercepted by vegetation, become overland flow over the ground surface, infiltrate into the ground, flow through the soil as subsurface flow, or discharge into streams as surface runoff.In a given watershed, discharge of water is primarily from groundwater withdrawals for irrigation, ET where the water table is near land surface, overland flow (runoff), and seepage to streams and springs where the water table intersects the land surface. Recharge of water is primarily from precipitation; other sources of recharge are irrigation return flow and seepage from streams, canals, and reservoirs. Large amounts of the intercepted water and surface runoff return to the atmosphere through evaporation. Infiltrated water may percolate to deeper soil layers to recharge groundwater, and later emerge in springs, or as seepage into streams, to form

Figure 1 The hydrologic cycle showing different components of the hydrological process.

Figure 1 The hydrologic cycle showing different components of the hydrological process.

surface flow. Finally, this water may flow to the larger rivers and, eventually to the sea and/or evaporate into the atmosphere. Throughout this cycle, water is usually subject to evaporation of one kind. Types of vegetation, management and land use, and climatic conditions significantly affect ET, and therefore determine the amount of water lost through ET from a watershed.

In agroecosystems, it is important to have a water balance to protect the sustainability and productivity of the agroecosystems. Water-level declines may result in increased costs for groundwater withdrawals because of increased pumping lift and decreased well yields. Waterlevel declines also can affect groundwater availability, surface water flow, and near-stream habitat (riparian) areas, and other ecological systems. Therefore developing efficient and effective management strategies is crucial for protecting sustainability of efficient use of water resources, protecting habitat and environment, and preventing ground and surface water degradation.

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