WATER QUALITY MANAGEMENT IN CONNECTED CATCHMENT- SHALLOW LAKE SYSTEMS - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 28

predominant sources of phosphorous in bodies of water are agricultural and excessive fertilizers and
improperly disposed animal wastes.

Figure (2-5): The phosphorous cycle

Human Inputs to the Phosphorus Cycle
Human influences on the phosphate cycle come mainly from the introduction and use of commercial
synthetic fertilizers. The phosphate is obtained through mining of certain deposits of calcium
phosphate called apatite. Huge quantities of sulfuric acid are used in the conversion of the phosphate
rock into a fertilizer product called "super phosphate". Plants may not be able to utilize all of the
phosphate fertilizer applied; as a consequence, much of it is lost from the land through the water run-
off. The phosphate in the water is eventually precipitated as sediments at the bottom of the body of
water. In certain lakes and ponds this may be redissolved and recycled as a problem nutrient. Animal
wastes or manure may also be applied to the land as fertilizer. In certain areas very large feed lots of
animals, may result in excessive run-off of phosphate and nitrate into streams. Other human sources of
phosphate are in the outflows from municipal sewage treatment plants. Without an expensive tertiary
treatment, the phosphate in sewage is not removed during various treatment operations. Again an extra
amount of phosphate enters the water.

2.3. IRRIGATED WATERSHEDS AND CATCHMENT - LAKE SYSTEMS : WATER QUALITY ISSUES

The physically complex watersheds in general may have some common features such as similar
physical components of the watershed including the drainage catchment, downstream catchment
connection to lakes, lagoons or direct connection to sea or ocean. The geographic location, hydrology,
land use and associated pollution problems are considered the specific characteristics of a watershed.
Freshwater systems in these watersheds are influenced by changing land-use patterns in the whole
watershed area. The pattern and extent of cities, roads, agricultural land, and natural areas within a
watershed influences infiltration properties, transpiration rates, and runoff patterns, which in turn
impact water quantity and quality. For example, expanding agricultural land increases the load of
nutrients and fertilizers from crops that end up in rivers and streams, impacting the water quality and
the biodiversity of freshwater ecosystems. In addition, because most of the fertile lands are already
under agriculture use, the areas for agriculture expansion are areas that require irrigation and usually
other inputs such as fertilizers. In many cases these agricultural practices cause soil erosion, water
logging, salinization, and water pollution from runoff that reduce the capacity of the soil to produce
crops in a sustainable way and also damage freshwater ecosystems.