Review of global agricultural water use per country
Irrigation water withdrawal
Assessing the impact of irrigation on water resources requires an estimate of the water effectively withdrawal for irrigation, i.e. the volume of water extracted from rivers, lakes and aquifers for irrigation purposes. Irrigation water withdrawal normally far exceeds the consumptive use of irrigation because of water lost in its distribution from its source to the crops. The ratio between the estimated irrigation water requirements and the actual irrigation water withdrawal is often referred to as "irrigation efficiency". However, the use of the words "irrigation efficiency" is currently subject of debate (Perry and Kite, 2003). The word "efficiency" implies that that water is being wasted when the efficiency is low. This is not necessarily so. Unused water can be used further down-stream in the irrigation scheme, it can flow back to the river or it can contribute to the recharge of aquifers. It is for this reason that in this document the term "water requirement ratio" will be used to indicate the ratio between irrigation water requirements and the amount of water withdrawn for irrigation.
Data on country water withdrawal for irrigation have been collected through the AQUASTAT country surveys. (FAO, 1995b, 1997a, 1997b and 1999c). Data on water requirement ratios are generally not easily available at field, irrigation scheme or river basin levels. At country level, only very scattered and unreliable information is available.
FAOSTAT is an on-line database currently containing over 1 million records covering international statistics on several agricultural variables. The only variable regarding irrigated agriculture in the FAOSTAT-database is 'land under irrigation'. Unlike the AQUASTAT-database, the FAOSTAT database consists of yearly time-series of each variable. The information to fill the FAOSTAT database is collected from country statistics, or, when country statistics are not available, is estimated by FAO. For this study, the AQUASTAT-data on water withdrawal for irrigated agriculture has been estimated for 2000, by assuming that the relative change in water withdrawal equals the relative change of land under irrigation as calculated from FAOSTAT figures.
After converting the AQUASTAT figures on water withdrawal of irrigated agriculture towards the same year, water requirement ratios at country level have been calculated. In order to estimate national figures on water requirement ratios, the AQUASTAT figures on water withdrawal have been compared with the calculated figures on crop water requirements. In view of the fact that large uncertainties exist on the value of the country figures, a regional rather than national approach was used to assess water requirement ratios:
WRRr = IWRr / AWWr
| where: | |
| r | is the region |
| WRR | is the water requirement ratio for the region |
| IWR | is the total irrigation water requirement for the countries in the region |
| AWW | is the total agricultural water withdrawal for the region, obtained from the country surveys. |
Individual estimates of water requirement ratios by country have then been obtained on the basis of these regional ratios and adapted on the basis of expert judgement when felt necessary. Dividing the irrigation water requirements from the water balance model by the water requirement ratio results in revised estimations of water withdrawal for irrigated agriculture per country. The results are summarised for five regions in the table below:
Summary results of agricultural water use and comparison with water resources
Total renewable water resources (km3) |
Irrigation water requirements (km3) |
Water requirement ratio |
Water withdrawal for agriculture (km3) |
Water withdrawal as percentage of renewable water resources |
|
| Latin America | 13409 | 45 | 24% | 187 | 1% |
| Near East and North Africa | 541 | 109 | 40% | 274 | 51% |
| Sub-Saharan Africa | 3518 | 31 | 32% | 97 | 3% |
| East Asia | 8609 | 232 | 34% | 693 | 8% |
| South Asia | 2469 | 397 | 44% | 895 | 36% |
| 90 developing countries | 28545 | 814 | 38% | 2146 | 8% |
On average, for the 90 developing countries, it is estimated that the water requirement ratio was around 38 percent in the reference period 2000, varying from 25 percent in areas of abundant water resources (Latin America) to 40 percent in Near East/North Africa and 44 percent in South Asia where water scarcity calls for higher water requirement ratios.
Irrigation water withdrawal was estimated to account for only 8 percent of total renewable water resources for the 90 countries. However, there are wide variations between regions, with the Near East / North Africa region using 53 percent of its water resources in irrigation while Latin America barely uses 1 percent. At the country level, variations are even higher: 10 countries used more than 40 percent of their water resources for irrigation in the reference year, a situation which can be considered critical. An additional nine countries used more than 20 percent of their water resources, a threshold that could be used to indicate impending water scarcity.
For several countries, relatively low national figures may give an overly optimistic impression of the level of water stress: China, for instance, is facing severe water shortage in the north while the south still has abundant water resources. Already by 2000, two countries (Libya, Saudi Arabia) used volumes of water for irrigation which were several times larger than their annual water resources. Groundwater mining also occurs at the local level in several other countries of the Near East, South and East Asia, Central America and in the Caribbean, even if at the national level the water balance may still be positive.
The results for individual countries are presented in this table.
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Conclusions |
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