Shifting dynamics of water use: nonlinear decline of transpiration-to-precipitation ratios with aridity

The amount of precipitation used by plants is an essential quantity for assessing ecosystem water-use efficiency and forecasting vegetation responses to environmental change in arid and semi-arid regions. The ratio of annual transpiration to annual precipitation (f₀) is an important yet neglected parameter, required for the accurate estimation of potential maximum leaf area index, light absorption and gross primary production in water-limited environments. This study estimated transpiration using three methods: the Penman-Monteith equation, Fick’s law, and an energy-balance technique based on flux measurements. Results showed a consistent pattern whereby f₀ initially increases with aridity but eventually declines, peaking at around 0.5–0.6 when the aridity index – defined as the ratio of annual (Priestley-Taylor) potential evapotranspiration to P – lies in the range from 2 to 3. This finding establishes a non-linear relationship between water supply and ecosystem water use, and points to a need to incorporate adaptive f₀ values in ecosystem models.