Pan Evaporation to Lake Evaporation Calculator

A pan reading is an index of atmospheric evaporative demand, not a measurement of lake evaporation, and Kp is the bridge between them. The US Class A pan is about 1.21 m across and 0.25 m deep in unpainted galvanised steel, standing on a slatted wooden platform roughly 0.15 m above the ground, with the water held between about 5 and 7.5 cm below the rim and read daily with a hook gauge to 0.25 mm. It evaporates faster than open water because it holds almost no heat, and because sunlight striking its exposed sides conducts straight into the water. A pan therefore responds to today's weather, whereas a lake responds to weeks of accumulated heat. Kp usually falls between 0.60 and 0.80, with 0.70 the conventional annual figure. FAO-56 tabulates values from roughly 0.35 to 0.85 as a function of the upwind land fetch, mean relative humidity, and wind run: dry, windy, advective sites sit at the low end, humid calm sites at the high end. The main misuse is applying one annual coefficient month by month. In a deep lake, heat storage delays peak evaporation into late summer and autumn, so the true monthly ratio can drop below 0.5 in spring and exceed 1.2 in autumn even when annual totals reconcile well. Siting matters too, since a pan on irrigated grass in an arid region receives far less advected sensible heat than one on bare dry soil nearby. For reservoirs deeper than about 10 m, energy-budget or Penman-type methods are preferred to any pan coefficient.

Open pans were used to gauge evaporation throughout the nineteenth century in an assortment of sizes and exposures - buried, floating, and elevated - which made records from different stations impossible to compare. The US Weather Bureau standardised the elevated Class A design in the early twentieth century, and it was later adopted internationally as a World Meteorological Organization reference instrument. Other national standards persisted in parallel, including the sunken Colorado pan and the Soviet GGI-3000, each carrying its own conversion coefficient. A physical basis for correcting pan data arrived in 1948, when Howard Penman combined an energy budget with an aerodynamic vapour-transfer term to compute open-water evaporation from ordinary weather observations. Calibration against a real water body came with the Lake Hefner studies in Oklahoma in 1950 and 1951, a joint effort of the US Geological Survey, the Navy, and the Weather Bureau, which measured reservoir evaporation by independent energy-budget and mass-transfer methods and supported a Class A coefficient near 0.70. Kohler, Nordenson, and Fox consolidated those findings in Weather Bureau Research Paper 38 in 1955. FAO Irrigation and Drainage Papers 24 in 1977 and 56 in 1998 published the Kp tables in the form most practitioners now use. Since the 1990s the pan evaporation paradox, the widespread decline in measured pan evaporation despite warming reported by Peterson and colleagues in 1995, has been attributed largely to falling wind speeds and reduced solar radiation rather than to any decline in lake evaporation itself.