three Water sources for flowing water fish culture

The hydrologic cycle

All the rivers run into the sea, yet the sea is not full; unto the place from whence the rivers come, thither they return again

Ecclesiastes 1:7

Mankind has long been aware of the hydrologic cycle—the processes of evaporation, transportation, condensation, precipitation, percolation, and runoff—by which water is recycled through the hydrosphere.

Evaporation occurs when radiant heat is absorbed and converted to kinetic energy in water molecules. The resulting molecular velocity causes some molecules to leave the water surface and enter the atmosphere. The energy lost in this process is the latent heat of vaporization, and the water surface cools. In addition to solar radiation, the rate of evaporation is affected by humidity, which determines the vapor pressure gradient; water turbidity, which may affect the amount of heat absorbed at the water surface; and, most importantly, by wind, which carries away the saturated layer of air at the water's surface, maintaining the vapor pressure gradient. Plants increase the surface area over which evaporation occurs. The evapotranspiration rate from aquatic plants may be twice as high as evaporation from the free water surface.

Condensation is caused by temperature changes in the air that affect its ability to hold moisture. The dew point is the temperature at which air becomes saturated with water. As air rises, it expands and cools according to the fundamental gas laws (see Chapter 5). When the dew point is reached, condensation occurs and clouds form. The clouds are made up of small droplets of liquid water too small to fall as rain. Precipitation occurs when ice crystals, dust, or sea salts act as hydroscopic nuclei upon which water droplets coalesce into raindrops. Air rises, causing precipitation by convective warming, by moving wedges of cold air that cause frontal storms, or by orographic processes in which air rises over geographic features.

Some rainfall is intercepted by plants and other obstacles and may evaporate before reaching the soil surface. The remainder enters the soil by percolation or runs off the soil surface into streams. The percolation rate is affected by gravity, capillarity, soil moisture content, and the amount of air in the soil pores. The layer of soil that retains soil moisture is called the zone of rock fracture. When this zone is saturated with water, infiltration continues to the zone of rock storage, where it is referred to as groundwater.

Stream flow consists of runoff, channel precipitation, and base flow, minus water losses to consumption, evaporation, and seepage. Base flow is the contribution to stream discharge from groundwater. The amount of precipitation that runs off a particular watershed is referred to as the hydrologic response and is expressed as a percentage. The hydrologic response is calculated from a hydrograph, which measures direct runoff above base flow following a precipitation event and from rain gauges in the watershed, which measure total precipitation. Hydrologic response values may range from 1% to 75% and are related to soil permeability, bedrock porosity, slope, and vegetation. Mountainous areas with shallow soils and impermeable bedrock have high hydrologic responses because a relatively large fraction of the water that falls as precipitation enters streams. Low hydrologic responses occur in areas where most of the precipitation enters the groundwater due to low gradients, permeable soils, and porous bedrock, such as limestone or shale.

Over 97% of the water on Earth is in the oceans and over 77% of the remaining freshwater is locked in ice caps and glaciers. Some intensive aquacultures involve the pumping of seawater to rearing units on land, but the water supplies usually used for flowing water fish culture are relatively shallow groundwaters and surface waters, usually from streams. These sources comprise a relatively small fraction of the water circulating through the hydrologic cycle and, in the case of streams, a very transient one.

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