Wateruse efficiency

The quantification of the dependence of plant productivity on water resources may be viewed as the slope of the relationship of net primary production and the amount of water actually lost by transpiration (T) over the year as

NPP = WUEt x water supply x proportion of water used by plants, where the season-long water-use efficiency (WUEt) or transpiration efficiency is the ratio of biomass produced to the corresponding plant transpiration [in g (dry matter) kg-1 H2O or mmol C mol-1 H2O] (Jones, 2004b). Water supply is precipitation plus irrigation, if appropriate, or precipitation during the growing season plus water in the soil at the moment of sowing for annual crops.

Short-term variability in transpiration efficiency is dominated by physiological (stomatal conductance and photosynthesis) and meteorological variables (vapour pressure deficit of the air, wind). The transpiration efficiency tends to increase under moderate water stress, as a result of greater stomatal restriction on transpiration and a relatively less sensitive response of the photosynthetic apparatus. On the contrary, high vapour pressure deficit in the atmosphere causes a decline in WUEt because transpiration increases without concomitant change in photosynthesis (Jones, 2004b). This sets an upper limit for WUEt in any given climate. Reduced transpiration under high irradiance raises the risk of leaf temperature increasing above the optimum for metabolic activity or at least above the threshold that leads to irreversible leaf tissue oxidative stress. Additionally, water-use efficiency (WUE) may decrease under severe water stress, or when water deficits combine with high temperature and high light, due to inhibition of photosynthesis (Chaves et al., 2004; Jones, 2004b). This is also apparent at the whole canopy level, as for example, under the Mediterranean summer drought, where WUEt decreased with severe water deficits accompanied by a strong decline in carbon assimilation (Reichstein et al., 2002).

At the scale of ecosystems we can integrate both hydrological and physiological components and ecosystem level WUE (WUEe; Gregory, 2004) is defined as:

where E is the direct evaporation from plant and soil surfaces, T is transpiration, R is the liquid water run-off and D is drainage below the rooting zone. Since in hydrological analysis it is common to separate liquid from vapour fluxes, the use of water for biomass production has been historically considered as the ratio of NPP to evapotranspiration (T + E) (Rosenzweig, 1968; Lieth & Whittaker, 1975). While T represents the amount of water required for primary production, the other terms of the water balance are virtually non-productive. The proportion of water transpired in relation to evapotranspiration [T/(T + E)] is a measure of water-supply efficiency (Rockstrom, 2003).

Reflecting roughly the impact of physiological controls, WUEe (or rain use efficiency) tends to be maximum under limiting water supply (Huxman et al., 2004), as suggested by Figure 6.2. The great variability in the data is mainly because of species differences and plant metabolism (e.g. C3/C4), differences in nutrition and soil properties and rainfall seasonality. The trend line shown for forests indicates that with high water supply the non-productive fluxes of water become more important. This trend was also shown in a eucalyptus plantation where irrigation and fertilisation treatments were applied (Table 6.1). The treatments were irrigation to satisfy the evapotranspiration demand in summer (I), irrigation as in I plus fertilisers added according to plant needs (IL), no irrigation but with fertilisers added (F) and control plots (C) (Madeira et al., 2002). WUEe decreased substantially (80%) in well-watered as compared to rainfed plots in the normal rainfall year as shown in Table 6.1 (precipitation close to the average, 607 mm). In a previous wet year (precipitation 1200 mm) the differences between well-watered and rainfed plots were negligible (unpublished results), but WUEe was approximately 12% greater in the fertilised plots than in the non-fertilised plots, both rainfed and irrigated.

Figure 6.2 Total NPP (g m-2 year-1) versus precipitation (mm) across world biomes. The trend line was drawn for forest data only (original data from Olson et al. (2001)).

Precipitation (mm)

Figure 6.2 Total NPP (g m-2 year-1) versus precipitation (mm) across world biomes. The trend line was drawn for forest data only (original data from Olson et al. (2001)).

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