Reservoir evaporation and the water balance

A dam operator closes the month's books: inflow measured, releases measured, level change measured, and the numbers do not quite add up. Some of the missing water went into the ground, some into the air. Reservoir evaporation is the part that leaves no trace in any gauge, so it is estimated rather than measured. This article explains what drives it, what rates are typical in India, and how evaporation loss from a reservoir is calculated.

What drives evaporation from a reservoir?

Evaporation happens when water molecules escape from the surface faster than they return. The Central Water Commission (CWC) lists the controls as water-surface area, temperature, the difference in vapour pressure between the water surface and the air above it, wind, atmospheric pressure and water quality [1]. Higher temperature, drier air and stronger wind all raise the rate.

Two things set a reservoir apart from a field. It stores heat, so its evaporation peaks later in the season than that of a pan placed nearby [3]. And its loss is proportional to surface area, which in a hill reservoir changes with level.

What is a typical reservoir evaporation rate in India?

Annual evaporation over India ranges from about 50 to 75 cm in parts of the Himalaya to as much as 325 cm in west Rajasthan and the Jalgaon–Akola region, according to CWC maps built from Indian Meteorological Department (IMD) observatory records [1]. The Deccan Plateau sees 250 to 325 cm, and the Himalaya and their foot 60 to 150 cm [1]. For reservoirs and water bodies CWC's 1990 review put the average at 150 to 300 cm a year, and used 225 cm to estimate that existing storages, then about 12,000 km² in surface area, lose some 27,000 million m³ a year [1].

How is evaporation loss from a reservoir calculated?

There are four established routes, all described in IS 6939 (1992) and in the WMO Guide to Hydrological Practices (WMO-No. 168) [2][3].

1. Pan evaporation with a coefficient. A standard pan is read daily, and the result is multiplied by a pan coefficient to give lake evaporation.
2. Water budget. Evaporation is what remains after everything else is accounted for (next section).
3. Energy budget. The radiation and heat terms of the lake are measured, and the energy used in evaporation is computed. Under good conditions WMO expects errors of about 10 % in summer and 20 % in winter [3].
4. Combination and mass-transfer equations. Meteorological data (temperature, humidity, wind, radiation) feed an equation such as Penman's, which blends an energy term and a wind and humidity term [3].

Pan evaporation is the least expensive method and often gives good estimates of annual evaporation [3]. The Indian standard describes the Class A land pan: 1,207 mm in diameter and 254 mm deep, filled to within 51 mm of the rim [2].

The pan coefficient

A pan is not a lake. It has little heat storage and exchanges heat through its sides. So its reading is scaled by a pan coefficient, the ratio of lake to pan evaporation. CWC quotes 0.70 as the accepted standard for a Class A land pan [1]. WMO adds the physics: when the pan water is warmer than the air in humid seasons the coefficient may be 0.80 or higher; in dry seasons and arid areas it may be 0.60 or less; and 0.70 applies when water and air temperatures are equal [3]. Experiments quoted by CWC found coefficients anywhere from 0.2 to 1.5, depending on pan size, depth and location [1].

Why not use FAO-56 reference evapotranspiration?

The FAO-56 Penman–Monteith equation estimates evapotranspiration from a hypothetical grass surface 0.12 m high [4]. FAO considered an open-water surface as a reference but noted that differences in aerodynamic, vegetation and radiation characteristics make it hard to relate crop evapotranspiration to free-water evaporation, so grass was chosen instead [4]. Grass-reference values are therefore not lake evaporation. They need a lake-specific coefficient or a method built for open water.

Where does evaporation sit in the reservoir water balance?

Over a period, the volume change of storage equals what comes in minus what goes out. Written for evaporation:

E = P + I − O − Seepage − ΔS        (volumes over the period, m³)

P = rain on the water surface     I = inflow       O = releases and spills
Seepage = net loss to the ground  ΔS = change in storage (positive when level rises)
E over the lake area A gives depth: E (mm) = 1000 × E / A

This is the storage equation given by WMO and CWC, rearranged and with sign conventions made explicit [1][3]. WMO says it plainly: evaporation found this way is a residual and "may be subject to considerable error if it is small relative to other parameters". The method is "difficult and inaccurate under most conditions, particularly for short averaging periods", and the period should be not less than a week [3]. Seepage and bank storage are the hardest terms to measure [1][3].

Worked example (illustrative)

Take a reservoir with a surface area of 10 km² and 50 million m³ of storage, in May. Pan evaporation is 8 mm/day.

Lake evaporation  = 0.70 × 8        = 5.6 mm/day
Volume per day    = 0.0056 m × 10,000,000 m² = 56,000 m³/day (0.65 m³/s)
Over 30 days      = 1.68 million m³  (3.4 % of storage)
Coefficient 0.60 to 0.80:  1.44 to 1.92 million m³

Now suppose we try to recover evaporation from the balance over a week. Evaporation is 5.6 × 7 = 39 mm. Two errors show how fragile that is:

A 1 % flow error is excellent practice; in a hill catchment it is rarely achieved. The pan method, with its uncertain coefficient, is usually the more stable estimate, and the water balance serves as a check over long periods.

What does this mean for reservoir operation?

• Treat evaporation as a term, not a leftover. Estimate it from pan or weather data and use the balance to check inflow and release records.
• Level accuracy matters. The level change enters the balance directly, so millimetres count. See reservoir level monitoring and measurement uncertainty for water data.
• Use the area-capacity curve. Loss scales with surface area, which changes with level.

Key points

• Reservoir evaporation is estimated, not measured; CWC cites 150 to 300 cm a year for Indian water bodies [1].
• The common method is pan evaporation times a pan coefficient, about 0.70 for a Class A pan, but varying with climate [1][3].
• The water balance gives evaporation as a residual and is poor over short periods [3].
• FAO-56 grass-reference evapotranspiration is not open-water evaporation [4].
• Accurate level, inflow and release records make every other term more reliable.

Where ClimMaTech fits

Evaporation is estimated from the weather, but the rest of the balance has to be measured at the dam. At ClimMaTech we make radar water level sensors such as Tarang, an AWLR (Automatic Water Level Recorder) with ±1 mm accuracy under ideal conditions, that log reservoir level continuously. Our dam and reservoir monitoring page describes how level, inflow and release records can be kept together for the water balance.

References

1. Central Water Commission, Basin Planning and Management Organisation, Evaporation Control in Reservoirs, Government of India, 2006. cwc.gov.in
2. Bureau of Indian Standards, IS 6939:1992, Methods for determination of evaporation from reservoirs (first revision, reaffirmed 2002). law.resource.org
3. World Meteorological Organization, Guide to Hydrological Practices, Volume I: Hydrology – From Measurement to Hydrological Information, WMO-No. 168, 2008 edition, updated 2020, chapter 4 (Evaporation, evapotranspiration and soil moisture). community.wmo.int
4. R. G. Allen, L. S. Pereira, D. Raes, M. Smith, Crop evapotranspiration: guidelines for computing crop water requirements, FAO Irrigation and Drainage Paper 56, chapter 2, Food and Agriculture Organization, 1998. fao.org