Year of publication
2003
Excerpt
Green water is that fraction of rainfall that infiltrates into the soil and is available to plants. It includes soil water holding capacity and the continual replenishment of reserves by rainfall. Green water is the largest fresh water resource, the basis of rain-fed agriculture and all life on land; and yet it has received remarkably little attention in contrast to blue water– the fraction of water that reaches rivers directly as runoff or, indirectly, through deep drainage to groundwater and stream base flow.This review encompasses:
•The concept of green water: The distinction betweengreen and blue water; practical definition, restricting the concept to rain-fed conditions and linking the concept of green water with rainwater use efficiency.
•Physical principles of water storage in the soil: The climatic, soil physical and ecological factors that determine the amount of green water and its distribution across the landscape; and the global availability of fundamental data
•Data availability:-The FAO-Unesco Soil Map of the World, compiled 20-40 years ago, is the only harmonised geographic source; supporting soil physical and soil water data are restricted for tropical areas generally. The FAO-ISRIC SOTER database at scales from 1:1M to 1:5M is more robust and, now, available for half of the land area.
-Global and regional climatic data are readily available, though of variable observation density and rarely including rainfall intensity, which is crucial to the partitioning of rainfall between runoff and infiltration. Data for decad or pentad periods and derived data such as length of growing season are most appropriate for determining the green water resource; temporal analysis is necessary to take account of season-to-season variability and long-term climatic trends.
-For up-to-date land cover data, interpretations of satellite imagery are the only realistic approach.
There are various experimental measurements of runoff on well-characterised sites; these provide some basis for extrapolation by soil, terrain and climatic units.
-Data availability remains a constraint.
•The concept of green water: The distinction betweengreen and blue water; practical definition, restricting the concept to rain-fed conditions and linking the concept of green water with rainwater use efficiency.
•Physical principles of water storage in the soil: The climatic, soil physical and ecological factors that determine the amount of green water and its distribution across the landscape; and the global availability of fundamental data
•Data availability:-The FAO-Unesco Soil Map of the World, compiled 20-40 years ago, is the only harmonised geographic source; supporting soil physical and soil water data are restricted for tropical areas generally. The FAO-ISRIC SOTER database at scales from 1:1M to 1:5M is more robust and, now, available for half of the land area.
-Global and regional climatic data are readily available, though of variable observation density and rarely including rainfall intensity, which is crucial to the partitioning of rainfall between runoff and infiltration. Data for decad or pentad periods and derived data such as length of growing season are most appropriate for determining the green water resource; temporal analysis is necessary to take account of season-to-season variability and long-term climatic trends.
-For up-to-date land cover data, interpretations of satellite imagery are the only realistic approach.
There are various experimental measurements of runoff on well-characterised sites; these provide some basis for extrapolation by soil, terrain and climatic units.
-Data availability remains a constraint.