Pond Evaporation & Water Loss: Calculating Makeup Water Needs

Where Pond Water Goes

A pond is an open container. Water leaves through three primary mechanisms: evaporation from the surface, seepage through the bottom and sides, and overflow during storm events. Water enters through precipitation falling directly on the surface, runoff from the surrounding watershed, groundwater inflow if the water table is above the pond bottom, and makeup supply from wells, pipes, or other sources.

A water balance accounts for all inputs and outputs: Inflow (precipitation + runoff + groundwater + makeup) minus Outflow (evaporation + seepage + overflow + withdrawal) equals Change in Storage. When the balance is negative over time, the pond level drops. The goal of a makeup water calculation is to determine how much additional inflow is needed to keep the balance at zero or positive.

For most constructed ponds without large watershed contributions, evaporation and seepage are the dominant losses. Precipitation partially offsets evaporation (rain falls on the pond surface), but in arid and semi-arid climates, evaporation far exceeds precipitation for most of the year. In humid climates, the net loss (evaporation minus precipitation) may be small or even negative during wet months, but summer months still show significant net evaporation.

Overflow is generally not a loss concern unless the pond has no spillway and overtops its banks, which is a dam safety issue rather than a water balance issue. Withdrawal (irrigation pumping, process water use) is a planned outflow that the operator controls.

Evaporation Physics: The Pan Coefficient

Evaporation from a water surface is driven by four factors: temperature (warmer water evaporates faster), humidity (drier air absorbs more moisture), wind (moving air carries moisture away from the surface), and solar radiation (energy input that drives the phase change from liquid to vapor). These factors interact in complex ways that make theoretical calculation difficult, which is why empirical measurement using evaporation pans has been the standard method since the 1800s.

The Class A evaporation pan is a 47.5-inch diameter, 10-inch deep galvanized steel pan set on a wooden platform at a weather station. Water is added daily to maintain a constant level, and the amount added is recorded as the daily pan evaporation rate. Thousands of weather stations across the United States have collected pan evaporation data for decades, providing a dense network of measurements.

However, a small metal pan evaporates faster than a large pond because the pan heats up more from solar radiation on its sides and bottom, and it has a higher perimeter-to-area ratio that exposes more water to wind. The pan coefficient (also called the pan factor) corrects for this difference. The standard value is 0.70, meaning a pond evaporates about 70 percent as much as the pan measurement at the same location.

The 0.70 coefficient is a national average derived from decades of comparative studies. Actual coefficients range from 0.60 in humid, sheltered coastal locations to 0.80 in arid, windy inland sites. If your state climatology office publishes lake evaporation data directly, use that instead of applying a pan coefficient. Lake evaporation data already accounts for the pan-to-lake correction and is more accurate for your specific region.

Climate Zones and Evaporation Rates

Annual free water surface evaporation in the contiguous United States ranges from about 20 inches in the Pacific Northwest to over 80 inches in the desert Southwest. This four-to-one range means that a pond in Phoenix loses four times as much water to evaporation as the same pond in Seattle. Understanding your climate zone is essential for estimating annual water loss.

Arid (Southwest): Annual pan evaporation of 80 to 120 inches, with lake evaporation of 56 to 84 inches. Summer months (June through August) account for 35 to 40 percent of the annual total. Monthly peak evaporation can exceed 10 inches in July. Ponds in this zone require significant makeup water year-round.

Semi-arid (Great Plains, Intermountain West): Annual pan evaporation of 50 to 80 inches, with lake evaporation of 35 to 56 inches. Summer peak is similar to arid zones but winter evaporation drops sharply due to lower temperatures and occasional ice cover. The growing season (April through October) accounts for 75 to 85 percent of annual evaporation.

Humid Continental (Midwest, Northeast): Annual pan evaporation of 30 to 50 inches, with lake evaporation of 21 to 35 inches. Precipitation roughly offsets evaporation on an annual basis, but summer months still show a net loss. Ponds in this zone may not need makeup water in spring and fall but do in summer.

Humid Subtropical (Southeast): Annual pan evaporation of 45 to 65 inches, with lake evaporation of 32 to 46 inches. High humidity reduces evaporation compared to the same temperature in a drier climate. Summer rainfall often exceeds monthly evaporation, but extended dry spells in late summer and fall can drop pond levels significantly.

Marine (Pacific Northwest, coastal areas): Annual pan evaporation of 20 to 35 inches, with lake evaporation of 14 to 25 inches. The lowest evaporation rates in the country. Precipitation far exceeds evaporation annually, and ponds in this zone rarely need makeup water except during unusual drought conditions.

Seepage Control: Liner Selection by Type

Seepage through the pond bottom and sides is the second major water loss mechanism, and unlike evaporation, it can be controlled through liner selection. The hydraulic conductivity of the bottom material determines how fast water moves through it. Published seepage rates by material type provide planning-level estimates.

Unlined native soil: Seepage depends entirely on soil type. Clean sand and gravel: 1 to 10 inches per day. Sandy loam: 0.5 to 2 inches per day. Silt loam: 0.1 to 0.5 inches per day. Clay: 0.01 to 0.1 inches per day. An unlined pond on sandy soil can lose more water to seepage than to evaporation. The only way to determine the actual rate is a seepage test: fill the pond, measure the water level drop over several days after accounting for evaporation, and calculate the seepage component.

Compacted clay liner: 6 to 12 inches of clay compacted to 95 percent of standard Proctor density reduces seepage to 0.01 to 0.1 inches per day. Clay liners are the most cost-effective option where suitable clay is locally available. They require careful compaction and must be protected from desiccation cracking (drying and cracking when exposed without water cover). Clay liners should not be used in ponds that are periodically emptied in hot weather.

Bentonite blanket: Sodium bentonite clay, either mixed into the native soil or applied as a geosynthetic clay liner (GCL), swells when wet and creates a low-permeability barrier. Seepage rates of 0.01 to 0.05 inches per day are typical. GCL products are factory-manufactured sheets that are rolled out like fabric and covered with soil. They are effective but sensitive to puncture, desiccation, and chemical attack from high-salinity or low-pH water.

HDPE geomembrane: High-density polyethylene liners (typically 30 to 60 mil thick) provide the lowest seepage rates: essentially zero when properly installed with welded seams. HDPE is the standard liner for hazardous waste ponds, landfill leachate ponds, and any application where zero seepage is required. The installed cost is $0.50 to $1.50 per square foot depending on thickness and site conditions. HDPE is resistant to UV, chemicals, and biological attack. Seams must be welded by certified installers and tested for integrity.

Concrete: Poured or shotcrete liners provide both structural support and seepage control. Seepage through sound concrete is near zero, but cracks from settling, thermal expansion, or structural loads can allow significant flow. Concrete liners are expensive ($3 to $8 per square foot) and are typically used only for small basins, water treatment structures, or applications requiring a hard surface for cleaning.

Building a 12-Month Water Budget

A water budget is a month-by-month accounting of all water gains and losses for the pond. It tells you which months require makeup water, how much, and at what flow rate. The budget also reveals whether your annual water cost justifies a liner upgrade or other loss-reduction investment.

For each month, calculate: Evaporation loss = surface area (sq ft) x monthly evaporation (inches) / 12 (to convert inches to feet) x 7.48 (gal per cu ft). Seepage loss = surface area x seepage rate (inches per day) x days in month / 12 x 7.48. Note that seepage also occurs through the sides, so for deep ponds with permeable soils, multiply the bottom-only seepage estimate by 1.3 to 1.5 to account for sidewall seepage.

On the input side: Precipitation gain = surface area x monthly rainfall (inches) / 12 x 7.48. Runoff gain depends on the contributing watershed area and soil type. For a pond with a small watershed (less than 5 times the pond area), runoff may be negligible. For larger watersheds, use a runoff coefficient (typically 0.1 to 0.4 for rural land) times the watershed area times monthly rainfall.

The net monthly loss = (evaporation + seepage) - (precipitation + runoff). Months where the net loss is positive require makeup water. The required makeup flow rate for each month = net monthly loss (gallons) / (days in month x 24 x 60) to get GPM. This tells you the continuous makeup flow needed to maintain a constant level.

Peak makeup demand typically occurs in July or August when evaporation is highest and rainfall may be lowest. Size your makeup water supply (pipe, pump, or water purchase agreement) to handle the peak month, not the annual average. A pond that needs 2 GPM average but 5 GPM in July will run short if the supply is sized for the average.

Makeup Pump and Supply Sizing

Once you know the peak monthly makeup demand in GPM, you can size the supply infrastructure. The options depend on the water source: municipal supply, on-site well, surface water diversion, or hauled water.

Municipal supply: A metered connection from the municipal water system is the simplest option for ponds near developed areas. Standard residential service (3/4-inch or 1-inch meter) delivers 10 to 30 GPM. A 2-inch commercial meter delivers 50 to 100 GPM. Water rates vary from $2 to $15 per 1,000 gallons depending on the municipality and rate tier. For large ponds, commercial rates and minimum charges apply. Some municipalities offer agricultural or non-potable water rates that are 30 to 50 percent lower than residential rates.

On-site well: A dedicated makeup well eliminates ongoing water purchase costs but requires upfront investment ($5,000 to $30,000 depending on depth and geology). Well capacity must match peak makeup demand. A well producing 10 GPM running 24 hours per day delivers 14,400 gallons per day, which covers the peak demand for most ponds under 2 acres. Permitting requirements vary by state; some states require a water appropriation permit for any new well.

Surface water diversion: Pumping from a nearby stream, river, or lake requires a water rights permit in most states. Western states use prior appropriation doctrine where water rights are allocated by seniority, and new diversions may not be available. Eastern states generally use riparian rights, which are more flexible but still require permits for significant diversions.

Hauled water: Water delivery by tanker truck is the most expensive option ($50 to $200 per load of 3,000 to 6,000 gallons) but requires no infrastructure. It is practical only for small ponds or emergency top-offs. At $100 per 4,000-gallon load, filling a 100,000-gallon deficit costs $2,500.

Cost Reduction Strategies

The two levers for reducing annual makeup water cost are reducing evaporation and reducing seepage. Evaporation is harder to control because it depends on climate, but seepage can be nearly eliminated with the right liner.

Liner upgrades provide the biggest single reduction in water loss for ponds on permeable soils. An unlined pond on sandy loam losing 1 inch per day to seepage loses about 27,000 gallons per day per acre. Installing a compacted clay liner reduces that to 270 gallons per day. An HDPE liner reduces it to near zero. At $5 per 1,000 gallons for municipal water, the unlined pond costs $49,000 per year in seepage makeup alone. The clay liner reduces that to $490, and HDPE reduces it to essentially zero. A $40,000 HDPE liner installation pays for itself in less than one year.

Evaporation reduction methods include floating covers, shade structures, and windbreaks. Floating black polyethylene balls or foam panels reduce evaporation by 70 to 90 percent but are practical only for small reservoirs and industrial tanks. Shade cloth structures reduce evaporation by 30 to 50 percent but are expensive for large ponds. Windbreaks (tree lines, fence rows) reduce wind speed across the pond surface and can reduce evaporation by 10 to 25 percent depending on the prevailing wind direction and fetch distance.

Deepening the pond reduces the surface-area-to-volume ratio. A deep, narrow pond loses less water per gallon stored than a shallow, wide pond because the evaporation surface is smaller relative to the volume. If you need to store 1 million gallons and can choose between a 1-acre pond at 3 feet deep and a 0.5-acre pond at 6 feet deep, the deeper pond loses half as much water to evaporation annually.

Water reuse reduces the need for makeup from external sources. Capturing roof runoff, parking lot runoff (with appropriate treatment), or recycling process water back to the pond offsets evaporation and seepage losses without increasing the water bill.