Glossary of terminology used
in the water resources survey and in the country water balance sheets

Note: Calculation rules are provided in the country water balance sheet

Internal renewable water resources (IRWR) in km3/year or 109m3/year

Average precipitation: Long-term double average over space and time of the precipitation falling on the country in a year, expressed in depth (mm/year) or in volume (km3/year or 109m3/year).

Surface water produced internally: Long-term average annual volume of surface water generated by direct runoff from endogenous precipitation (surface runoff).

Groundwater produced internally: Long-term annual average groundwater recharge, generated from precipitation within the boundaries of the country. Renewable groundwater resources of the country are computed either by estimating annual infiltration rate (in arid countries) or by computing river base flow (in humid countries).

Overlap between surface water and groundwater: Part of the renewable water resources which is common to both surface water and groundwater. It is equal to groundwater drainage into rivers (typically, base flow of rivers) minus seepage from rivers into aquifers.

Total internal renewable water resources: Long-term average annual flow of rivers and recharge of aquifers generated from endogenous precipitation. Double counting of surface water and groundwater resources is avoided by deducting the overlap from the sum of the surface water and groundwater resources.

Water resources produced internally in a 10th dry year frequency: The water resources that are "guaranteed" in a dry year of a tenth frequency. This information is particularly important in arid and semi-arid climate, as dry year resources may be less than half the annual average flow.

External renewable water resources (ERWR) in km3/year or 109m3/year

External renewable surface water resources

Inflow not submitted to agreements or treaties: Long-term average quantity of water annually entering the country through transboundary flow (rivers, canals, pipes). This figure concerns only the flows which are not submitted to formal agreements or treaties.

Inflow submitted to agreements or treaties: Average quantity of water annually entering the country which is submitted to but not yet secured through formal agreements or treaties.

Inflow secured through agreements or treaties: Average quantity of water annually entering the country which is secured through formal agreements or treaties.

Accounted natural inflow: Long-term average quantity of water annually entering the country under natural conditions, i.e. not affected by or before being affected by upstream consumption.

Accounted actual inflow: The sum of the average quantities of water annually entering the country not submitted to treaties and the quantities secured through treaties.

Total natural flow of border rivers: Long-term average annual total natural flow of rivers that from the border between countries.

Total actual flow of border rivers: Average annual flow of rivers that form the border between countries, taking into account water abstraction from upstream countries and/or agreed or accepted commitments towards downstream countries.

Accounted natural flow of border rivers: As a general rule, 50 percent of the total river flow is assigned to each of the bordering countries. Several situations exist: (i) Where the river exclusively borders the countries without entering any of the adjacent countries nor exiting from them, the incoming resources are estimated on the basis of the river runoff in the upstream part of the border section; where the runoff increases substantially from upstream to downstream, the downstream figure is used after subtraction of the part of the runoff generated by the country itself; (ii) Where the source of the river is in one of the two countries, the rule applies only for the other country; for the originating country, 50 percent of the contribution from the other country could similarly be considered as external resources where known; (iii) Where the river enters one of the two countries after having divided the two countries, it is considered a transboundary river for the receiving country and all the runoff at the entry point in that country is considered as an external resource; the 50-percent rule applies for the other country.

Accounted actual flow of border rivers: As a general rule, 50 percent of the total river flow is assigned to each of the bordering countries. The same rules apply as described under the previous definition (natural), but where a treaty exists between the adjacent countries of a river system, the rules applied are those defined in that treaty.

Accounted natural part of shared lakes: Part of the quantity of water of a lake attributed to the country. Several situations exists: (i) Where the lake has an outlet into a river (e.g. Lake Victoria enters the Nile River in Uganda), all the runoff at the entrance of the river is accounted for as external resources for the receiving country; for all the other countries, an equal share of this runoff can be considered as external resources, after having subtracted the country contribution to the lake; where this results in a negative value, the external resources are considered to be zero for the country in question; where the river forms the border between two countries, the rule described for border rivers applies; (ii) For lakes without an outlet, the global runoff entering the lake is estimated and shared equally between the adjacent countries, after having deducted the part contributed from the country; where this results in a negative value, the external resources are considered to be zero for the country in question; (iii) Artificial lakes have not been accounted for as the flow reduction is an impact of water development and not a natural phenomenon.

Accounted actual part of shared lakes: Same rules apply as described under the definition above (natural), but where a treaty exists between the adjacent countries of the lake, the rules applied are those defined in that treaty.

Surface water leaving the country: This is the sum of the average quantities of water annually leaving the country not submitted to treaties and secured through treaties. The computation of actual ERWR considers the outflow of surface water only in the case of an agreed apportionment between the upstream and downstream countries.

Outflow not submitted to agreements or treaties: Average quantity of water annually leaving the country (including to the sea) and not submitted to treaties.

Outflow to be reserved through agreements or treaties: Average annual quantity of water reserved by treaty for a downstream country.

Total natural external renewable surface water: The sum of the accounted inflow, the accounted flow of border rivers and the accounted part of shared lakes.

Total actual external renewable surface water: The sum of the actual inflow not submitted to treaties, actual inflow secured through treaties, the accounted flow of border rivers and the accounted part of shared lakes, minus the outflow of water reserved for downstream countries through treaties.

External renewable groundwater resources

Groundwater entering the country or external groundwater: Average quantity of groundwater annually entering the country (usually restricted to large aquifers shared by several arid countries).

Groundwater leaving the country or outflow into a neighbouring country: Average quantity of groundwater annually leaving the country to other countries (usually restricted to large aquifers shared by several arid countries).

Total external renewable water resources

Total natural external renewable water resources (ERWRnatural): The sum of the total natural external surface water resources and the external groundwater resources.

Total actual external renewable water resources (ERWRactual): That part of the country's annual renewable water resources which is not generated in the country. It includes inflows from upstream countries (groundwater and surface water), and part of the water of border lakes or rivers. Contrary to natural external renewable water resources (i.e. the situation without human influence), ERWRactual take into account the quantity of flow reserved by upstream (incoming flow) and/or downstream (outflow) countries through formal or informal agreements or treaties, and possible water abstraction occurring in the upstream countries. Therefore, it may vary with time.

Total renewable water resources (TRWR) in km3/year or 109m3/year

Total natural renewable surface water: The sum of the internal renewable surface water resources and the total external natural renewable surface water resources.

Total actual renewable surface water: The sum of the internal renewablesurface water resources and the total external actual renewable surface water resources.

Total natural renewable groundwater: The sum of the internal renewable groundwater resources and the total external natural renewable groundwater resources.

Total actual renewable groundwater: The sum of the internal renewable groundwater resources and the total external actual renewable groundwater resources. In general natural and actual external (entering) renewable groundwater resources are considered to be the same.

Total natural renewable water resources (TRWRnatural): The sum of internal renewable water resources (IRWR) and external natural renewable water resources (ERWRnatural). It corresponds to the maximum theoretical yearly amount of water actually available for a country at a given moment. Natural resources are considered stable over a long period.

Total actual renewable water resources (TRWRactual): The sum of internal renewable water resources (IRWR) and external actual renewable water resources (ERWRactual), which take into consideration the quantity of flow reserved to upstream and downstream countries through formal or informal agreements or treaties and possible reduction of external flow due to upstream water abstraction. It corresponds to the maximum theoretical yearly amount of water actually available for a country at a given moment. While natural resources are considered stable over time, actual resources may vary with time and refer to a given period.

Dependency ratio (%): That part of the total renewable water resources originating outside the country.

Exploitable water resources in km3/year or 109m3/year

Exploitable regular renewable surface water resources: Annual average quantity of surface water that is available with an occurrence of 90 percent of the time. In practice, it is equivalent to the low water flow of a river. It is the resource that is offered for withdrawal or diversion with a regular flow.

Exploitable irregular renewable surface water resources: Irregular surface water resources are equivalent to the variable component of water resources (e.g. floods). It includes the seasonal and inter-annual variations, i.e. seasonal flow or flow during wet years. It is the flow that needs to be regulated.

Exploitable regular renewable groundwater resources: Annual average quantity of groundwater that is available with an occurrence of 90 percent of the time. It is the resource that is offered for groundwater extraction with a regular flow.

Total exploitable or manageable water resources: That part of the water resources which is considered to be available for development under, taking into consideration factors such as: the economic and environmental feasibility of storing floodwater behind dams or extracting groundwater, the physical possibility of catching water which naturally flows out to the sea, and the minimum flow requirements for navigation, environmental services, aquatic life, etc. It is also called water development potential. Methods to assess exploitable water resources vary from country to country depending on the country's situation. In general, exploitable water resources are significantly smaller than natural water resources.

Other terms used

Natural flow: The amount of water which would flow in natural conditions, i.e. without human influence.

Potential yield: Total amount of water resources, be it surface water or groundwater, generated by the hydrological cycle on a yearly basis. It is a physical concept, and is equivalent to the natural resources.

Blue water: Equivalent to the natural water resources (surface water and groundwater runoff).

Green water: The rainwater directly used and evaporated/transpired by non-irrigated agriculture, pastures and forests.

Evapotranspiration: The actual rate of water uptake by the plant which is determined by the level of available water in the soil.

Evaporation from reservoirs and lakes: Annual amount of water evaporated from large reservoirs and lakes. The evaporation to take into account is the net evaporation (evaporation minus precipitation). A portion of flow generated internally is not entirely collected by the hydrological system. With regard to the evaluation of resources, evaporation from wetlands, lakes and rivers is a reducing factor of the entire flow generated. However this reduction of runoff should not be considered entirely as a "loss" because it corresponds partly to the needs of nature, specifically those needs of aquatic systems.

Renewable water resources: The long-term average annual flow of rivers (surface water) and recharge of aquifers (groundwater) generated from precipitation. They are computed on the basis of the water cycle.

Non-renewable water resources: Groundwater bodies (deep aquifers) that have a negligible rate of recharge on the human time-scale and thus can be considered as non-renewable. While renewable water resources are expressed in flows, non-renewable water resources have to be expressed in quantity (stock).

Secondary water resources: It refers to the return of primary water in the system, thus becoming available again for exploitation. In fact, it is an interaction between resources and utilization in a same area, without increasing the natural resource. Statistics on secondary resources can be useful for the complete comparison between resources and utilization.Secondary water resources can be considered as a type of non-conventional sources of water.

Non-conventional sources of water: They include: (i) The production of freshwater by desalination of brackish water or saltwater; (ii) The reuse of urban or industrial wastewaters (with or without treatment), which increases the overall efficiency of use of water (extracted from primary sources). They are accounted for separately from natural renewable water resources.

Agricultural drainage water: Water withdrawn for agriculture but not consumed and returned. It does not go through special treatment and therefore should be distinguished from reused wastewater. It can be considered as secondary or non-conventional water.