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The land used effects to hydrologic cycle

Introduction

The hydrologic cycle describes the continuous movement of water above, on,

and below the surface of the Earth. The water on the Earth's surface occurs as

streams, lakes, and wetlands, as well as bays and oceans. Surface water also

includes the solid forms of water example snow and ice. The water below the surface

of the Earth primarily is ground water, but it also includes soil water.

Figure 1:The stages of the Hydrological Cycle

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Effects

Changing land use and land management practices can also alter the

hydrological system. Land-use change has a direct effect on hydrologic processes

through its link with the evapotranspiration regime on one hand and the sometimes

extreme change in surface runoff on the other hand.

Hence, land use also plays a main role in assessing groundwater resources.

Precipitation is the primary source of groundwater and varies both temporally and

spatially.

For most hydrological applications, it is appropriate to assume that

precipitation is independent of vegetation type, and evapotranspiration and surface

runoff are closely linked with land-cover characteristics. A large number of land-use

impact studies on water resources have been carried out for watersheds with a focus

on water scarcity, flood, erosion, and water management.

The combined effect of climate and land-use changes shows that

deforestationleads to increased groundwater recharge in arid areas, because

deforestation leads to reduced evapotranspiration eve n though it favors runoff.

Figure 2:Groundwater flow patterns and the zone of dispersion in an idealized,

homogeneous coastal aquifer

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The amount of precipitation that soaks into the soil depends on several

factors:

y the amount and intensity of the precipitation,

y the prior condition of the soil,

y the slope of the landscape

y the presence of vegetation.

These factors can interact in sometimes surprising ways:

y a very intense rainfall onto very dry soil

y typical of the desert southwest

y often will not soak into the ground at all

y creating flash-flood conditions.

Water that does soak in becomes available to plants. Plants take up water

through their root systems, the water is then pulled up through all parts of the plant

and evaporates from the surface of the leaves, a process called transpiration. Water

that soaks into the soil can also continue to percolate down through the soil profile

into groundwater reservoirs, called aquifers. Aquifers are often mistakenly visualized

as great underground lakes; in reality, groundwater fills the pore spaces within

sediments or rocks.

Figure 3:Groundwater exists below the water table, which divides unsaturated soil,

rock, and sediments from saturated.

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Water that doesnt soak into the soil collects and moves across th e surface as

run-off, eventually flowing into streams and rivers to get back to the ocean.

Precipitation that falls as snow in glacial regions takes a somewhat different journey

through the water cycle, accumulating at the head of glaciers and causing them to

flow slowly down valleys.