Development of Functional Irrigation Types for Improved Global Crop Modelling
J. Rohwer, D. Gerten, W. Lucht (March 2007)
About 70% of global human water withdrawal from rivers, reservoirs, and aquifers is for irrigation of agricultural land, and about one third of global food production relies on irrigation water. Irrigation systems, however, are usually rather ineffective; much of the withdrawn water is lost before it reaches the plants that require the water for optimal growth. The efficiencyof irrigation often is lower than half of the optimum (depending e.g. on climate andirrigation system), such that the accuracy of modelled efficiency may have strong effects on the simulated water cycle and the distribution, seasonal phenology, and productivity (yields) of crop types. Therefore, it is crucial to estimate irrigation efficiencies as detailed as possible in any (large-scale) irrigation assessment.
In this study, generic irrigation functional types (IFTs) have been developed for a better representation of irrigation in the dynamic global vegetation and water balance model including managed land, LPJmL (Bondeau et al., 2006). LPJ is a model of intermediate complexity, representing the intra- and interannual dynamics of terrestrial vegetation (both natural and agricultural) and the associated biophysical and biogeochemical processes (e.g. carbon and water fluxes). Irrigation of agricultural vegetation is one important option of land and water management in this model. The IFTs developed here improve the way in which irrigation efficiencies have been considered in the pilot version of LPJmL. With the introduction of IFTs into LPJmL together with the recently implemented discharge accumulation along a global river topology (Jachner et al., submitted), water flows on agricultural land are represented in a more precise manner.
The present country-scale IFT classification was derived from the dominant irrigation method (surface, sprinkler, or micro-irrigation), irrigation field size, and the associated management system, based on an extensive review of literature and data archives. We determine an overall irrigation project efficiency for each country. This overall efficiency is comprised by a combination of individual (partial) efficiencies that capture water losses a) from the conveyance systems (conveyance efficiency), b) when the irrigation water is brought to the field (field application efficiency), and c) a management factor determined by the irrigation system size, which is a substitute of distribution efficiency (the unequal distribution of irrigation water across the fields). The effectiveness in terms of timeliness of delivery is represented by application of scheduling rules. The dominant irrigation method is used to assign the respective IFT to each country; for a few countries where relevant data were not available, a statistical assignment procedure based on socio-economic and climatic information was employed. The resulting country-scale irrigation efficiencies are tabulated and presented in global maps. Our efficiencies compare well with earlier values that had been derived for larger regions; yet, the present study provides more detailed and consistent estimates of irrigation efficiencies, which owing to their generic nature are suited for application in any large-scale model that represents agricultural water use.
