Precipitation, Evapotranspiration, and the Energy Balance

Introduction

From orographic uplift to the Penman-Monteith equation: where water enters and leaves the land surface

Precipitation begins when water vapour in rising air cools to the dewpoint and condenses onto tiny aerosol particles — condensation nuclei — to form cloud droplets that grow until they fall. Three main mechanisms lift air to the dewpoint: convective lifting (instability-driven; typical of tropical thunderstorms and afternoon showers), orographic lifting (forced ascent over mountain barriers; produces 5,000+ mm/yr in windward ranges like the Western Ghats and Hawaiian Ko'olau), and frontal/cyclonic lifting (lifting along weather fronts and within extratropical cyclones). Global mean precipitation is about 990 mm/yr over land, but with extreme variability: the Atacama Desert receives < 0.1 mm/yr while Cherrapunji, India received 26,470 mm (1042.12 in) in a single year (1861).

Evapotranspiration (ET) is the combined flux of water vapour from land surfaces — including direct evaporation from soil, open water, and wet canopies, plus transpiration through plant stomata. ET accounts for ~73,000 km³ (17,513 cu mi)/yr globally — about 65% of all land precipitation returns directly to the atmosphere this way. In humid climates ET is energy-limited; in arid climates it is water-limited. The Penman-Monteith equation (1948, adopted as FAO standard in 1998) is the accepted method for calculating reference ET from meteorological variables — net radiation, air temperature, wind speed, and vapour pressure deficit — and forms the basis for irrigation scheduling worldwide.

The water balance equation links these fluxes: precipitation (P) equals evapotranspiration (ET) plus runoff (Q) plus the change in storage (ΔS), or P = ET + Q + ΔS. Over long time periods, storage change averages to near zero and the equation simplifies to: precipitation is partitioned between what evapotranspires and what runs off. Globally, about 65% of land precipitation evapotranspires and ~35% becomes river discharge. The Budyko framework describes this partitioning as a function of the aridity index (PET/P): in humid catchments the ratio of actual ET to precipitation approaches 1.0; in hyper-arid ones it falls toward 0.

Remote sensing has transformed precipitation and ET measurement. NASA's Global Precipitation Measurement (GPM) mission (launched 2014, successor to TRMM) provides near-global precipitation estimates at 0.1° × 0.1° resolution every 30 minutes by combining active radar measurements (DPR) with passive microwave radiometry from a constellation of satellites. For ET, eddy covariance flux towers (FLUXNET network: >900 sites globally) measure vertical turbulent fluxes of water vapour and CO₂ directly, while remote sensing algorithms (MODIS MOD16, SSEBop, SEBS) estimate ET from land surface temperature, vegetation indices, and meteorological reanalyses at regional to global scales.

Key Terms