Water resources are sources of
water that are useful or potentially useful to
humans. Uses of water include
agricultural,
industrial,
household,
recreational and
environmental activities. Virtually all of these human uses require
fresh water.
97% of water on the Earth is salt water, and only 3% is fresh water of which slightly over two thirds is frozen in
glaciers and
polar ice caps.
[1] The remaining unfrozen freshwater is mainly found as groundwater, with only a small fraction present above ground or in the air.
[2]
Fresh water is a
renewable resource, yet the world's supply of clean, fresh water is steadily decreasing. Water demand already
exceeds supply in many parts of the world and as the
world population continues to rise, so too does the water demand. Awareness of the global importance of preserving
water for
ecosystem services has only recently emerged as, during the 20th century, more than half the world’s
wetlands have been lost along with their valuable environmental services.
Biodiversity-rich
freshwater ecosystems are currently declining faster than
marine or land
ecosystems.
[3] The framework for allocating water resources to water users (where such a framework exists) is known as
water rights.
Surface Water
Surface water is water in a river,
lake or fresh water
wetland. Surface water is naturally replenished by
precipitation and naturally lost through discharge to the
oceans,
evaporation,
evapotranspiration and sub-surface seepage.
Although the only natural input to any surface water system is precipitation within its
watershed, the total quantity of water in that system at any given time is also dependent on many other factors. These factors include storage capacity in lakes, wetlands and artificial
reservoirs, the permeability of the
soil beneath these storage bodies, the
runoff characteristics of the land in the watershed, the timing of the precipitation and local evaporation rates. All of these factors also affect the proportions of water lost.
Human activities can have a large and sometimes devastating impact on these factors. Humans often increase storage capacity by constructing reservoirs and decrease it by draining wetlands. Humans often increase runoff quantities and velocities by paving areas and channelizing stream flow.
The total quantity of water available at any given time is an important consideration. Some human water users have an intermittent need for water. For example, many
farms require large quantities of water in the spring, and no water at all in the winter. To supply such a farm with water, a surface water system may require a large storage capacity to collect water throughout the year and release it in a short period of time. Other users have a continuous need for water, such as a
power plant that requires water for cooling. To supply such a power plant with water, a surface water system only needs enough storage capacity to fill in when average stream flow is below the power plant's need.
Nevertheless, over the long term the average rate of precipitation within a watershed is the upper bound for average consumption of natural surface water from that watershed.
Natural surface water can be augmented by importing surface water from another watershed through a
canal or
pipeline. It can also be artificially augmented from any of the other sources listed here, however in practice the quantities are negligible. Humans can also cause surface water to be "lost" (i.e. become unusable) through
pollution.
Under River Flow
Throughout the course of the river, the total volume of water transported downstream will often be a combination of the visible free water flow together with a substantial contribution flowing through sub-surface rocks and gravels that underlie the river and its floodplain called the
hyporheic zone. For many rivers in large valleys, this unseen component of flow may greatly exceed the visible flow. The hyporheic zone often forms a dynamic interface between surface water and true ground-water receiving water from the ground water when aquifers are fully charged and contributing water to ground-water when ground waters are depleted. This is especially significant in
karst areas where pot-holes and underground rivers are common.
Sub-surface water, or groundwater, is fresh water located in the
pore space of soil and
rocks. It is also water that is flowing within
aquifers below the
water table. Sometimes it is useful to make a distinction between sub-surface water that is closely associated with surface water and deep sub-surface water in an aquifer (sometimes called "fossil water").
Sub-surface water can be thought of in the same terms as surface water: inputs, outputs and storage. The critical difference is that due to its slow rate of turnover, sub-surface water storage is generally much larger compared to inputs than it is for surface water. This difference makes it easy for humans to use sub-surface water unsustainably for a long time without severe consequences. Nevertheless, over the long term the average rate of seepage above a sub-surface water source is the upper bound for average consumption of water from that source.
The natural input to sub-surface water is seepage from surface water. The natural outputs from sub-surface water are
springs and seepage to the oceans.
If the surface water source is also subject to substantial evaporation, a sub-surface water source may become
saline. This situation can occur naturally under
endorheic bodies of water, or artificially under
irrigated farmland. In coastal areas, human use of a sub-surface water source may cause the direction of seepage to ocean to reverse which can also cause
soil salinization. Humans can also cause sub-surface water to be "lost" (i.e. become unusable) through pollution. Humans can increase the input to a sub-surface water source by building reservoirs or detention ponds.
