WATER QUALITY MANAGEMENT IN CONNECTED CATCHMENT- SHALLOW LAKE SYSTEMS - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 18

it is necessary to understand the hydrologic cycle and the circulation of water from the ocean,
atmosphere and land.
Human influences have had a direct effect on the hydrologic cycle by altering the land in ways that
change its physical, chemical, and biological characteristics (Hem, 1985; Meybeck and Helmer, 1989).
Physical alterations such as urbanization, transportation, farming (irrigation), deforestation and
forestation, land drainage, channelization and damming, and mining alter hydrologic pathways and
may change the water quality characteristics by modifying the materials with which the water
interacts. In addition, these human activities alter water quality not only by changing hydrologic
pathways, but by the addition of substances and wastes to the land. These activities include application
of pesticides, herbicides, and fertilizers, and leaching to groundwater and surface water from landfills,
mine tailings, and irrigated agricultural lands.
The chemical alteration associated with human activity is, in part, related to the physical alteration, but
occurs mainly through the addition of wastes (gases, liquids, and solids) and other substances to the
land. These additions include waste disposal on the land or in waterways and the application of
substances to control the environment, such as fertilizers for crop production, herbicides for weed
control, and pesticides for disease control. Atmospheric transport and deposition is a major hydrologic
pathway of substances directly to surface water or indirectly to groundwater by infiltration through the
soil. Human requirements for water also directly affect hydrologic pathways by providing water of a
specified quality for different activities to sustain human existence (e.g., agriculture, potable supplies,
power generation, power plant cooling, and industry).
Human activity is considered one of the most important factors affecting hydrology and water quality.
Humans use large amounts of resources to sustain various standards of living, although measures of
sustainability are highly variable depending on how sustainability is defined (Moldan et al., 1997).
Irrigated agriculture alone is responsible for about 75 percent of the total water withdrawn from
“surface water and groundwater sources,” and more than 90 percent of this water is consumed and
delivered to the atmosphere by evaporation (United Nations Commission for Sustainable
Development, 1997). In addition to placing a demand on the quantity of water, which is diverted for
food production, the quality of water flowing through a typical agricultural area is markedly degraded.
Degradation depends on several factors including the climatic characteristics and the various fertilizers
and agrochemicals applied to increase yields. The excessive use of these fertilizers and agrochemicals
has long-term effects on ground and surface water resources.
Each water use, including abstraction of water and discharge of wastes, leads to specific, and generally
rather predictable, impacts on the quality of the aquatic environment. In addition to these intentional
water uses, there are several human activities which have indirect and undesirable effects on the
aquatic environment. Examples are uncontrolled land use for urbanisation or deforestation, accidental
(or unauthorised) release of chemical substances and discharge of untreated wastes or leaching of toxic
liquids from solid waste deposits into water bodies. It is also important to mention the structural
interventions in the natural hydrological cycle through canalisation or damming of rivers, diversion of
water within or among drainage basins. These interventions are usually undertaken with a beneficial
objective in mind but their long term impacts on the water quality is obvious through experiences and
cases all over the world.
Pollution Sources and Environmental Risks
Water quality reflects the composition of water as affected by nature and human cultural activities and
uses, expressed in terms of both measurable quantities and descriptive statements. The quality status of
receiving water bodies and their pollutants could be understood in a more comprehensive manner,
expressed as 'integrity'. Integrity of the water body in this sense means taking into consideration all
the affecting factors and parameters that directly impact the water quality or increase the pollution