Introduction
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
Precipitation caused by forced lifting of moist air over mountain barriers; creates dramatic wet/dry contrasts across ranges.
FAO standard method to calculate reference ET (ET₀) from net radiation, temperature, wind speed, and vapour pressure deficit.
Maximum possible evapotranspiration with unlimited water; energy-limited. Actual ET ≤ PET always.
Difference between saturation vapour pressure and actual vapour pressure of air; drives evapotranspiration.
Describes AET/P as a function of aridity index (PET/P); predicts how much precipitation evapotranspires vs runs off.