Farm Dam Calculator
Plan dam storage from dimensions, catchment runoff, rainfall, evaporation, stock water demand, irrigation demand, freeboard, usable storage, and days of reserve.
Use measured site data where possible. This planning calculator estimates storage and water balance; dam design, spillway capacity, seepage control, and legal approvals should be checked locally.
Farm Dam Planning Results
Results combine dam geometry, catchment runoff, evaporation loss, stock demand, irrigation draw, freeboard, and unusable reserve.
| Catchment surface | Runoff coefficient | Good planning use | Field note |
|---|---|---|---|
| Sandy pasture, heavy cover | 0.05 to 0.15 | Low refill estimate | Much rainfall soaks in before reaching the dam |
| Average grass catchment | 0.12 to 0.25 | Stock dam planning | Use the lower end in flat or highly permeable paddocks |
| Cultivated clay loam | 0.25 to 0.40 | Mixed farm storage | Residue cover and slope change runoff sharply |
| Compacted yard or laneway | 0.45 to 0.70 | Small high-yield catchment | Check sediment and water quality before stock use |
| Roof or hardstand capture | 0.75 to 0.90 | Reliable supplemental inflow | First-flush and debris screens may be needed |
| Livestock class | Typical daily water | Hot-weather allowance | Planning note |
|---|---|---|---|
| Beef cow | 12 to 20 gal/head | 25 to 35 gal/head | Lactation, salt, and heat raise demand |
| Dairy cow | 30 to 50 gal/head | 55 to 70 gal/head | Milk production drives high water use |
| Horse | 8 to 15 gal/head | 18 to 25 gal/head | Exercise and dry hay increase intake |
| Sheep or goat | 1 to 3 gal/head | 3 to 5 gal/head | Use flock totals plus trough loss |
| Finishing pig | 2 to 5 gal/head | 5 to 8 gal/head | Include washing and cooling where relevant |
| Irrigation target | Seasonal depth | Gallons per acre | Best planning fit |
|---|---|---|---|
| Emergency tree watering | 2 to 4 in | 54,000 to 109,000 | Keep trees alive during dry spells |
| Pasture boost | 4 to 8 in | 109,000 to 217,000 | Short rotation or targeted paddocks |
| Market vegetables | 8 to 14 in | 217,000 to 380,000 | High-value beds with scheduled irrigation |
| Orchard block | 10 to 18 in | 271,000 to 489,000 | Depends heavily on canopy and soil water |
| Field crop support | 6 to 12 in | 163,000 to 326,000 | Often supplemental, not full-season supply |
| Dam design item | Common planning range | Calculator input | Why it matters |
|---|---|---|---|
| Freeboard | 1.5 to 3 ft | Freeboard above full supply | Reduces overtopping risk during wind and storms |
| Inside side slope | 2:1 to 4:1 | Horizontal to 1 vertical | Controls bottom area and bank stability |
| Unusable reserve | 10% to 30% | Sediment and dead storage | Keeps intake depth and mud out of routine supply |
| Evaporation factor | 0.65 to 0.80 of pan | 70% used here | Approximates open-water loss from pan data |
| Spillway level | Full supply level | Top length and width | Capacity is calculated below this water surface |
For dependable stock water: Size the dam from usable storage, not gross capacity. Keep sediment, pump intake depth, and poor-quality bottom water outside the normal working supply.
For catchment estimates: Use local rainfall records and soil-specific runoff. A small change in runoff coefficient can move the annual balance by more than the stock demand.
When planning a dam, a person must considers the difference between the total volume of a dam and the usable volume of a dam. While many people consider the total volume of a dam, the total volume of a dam isnt the same as the usable volume of a dam. A dam can lose water to evaporation, sediment can fill the bottom of the dam, and there may be daily demands for the water in the dam from livestock or irrigation efforts.
Thus, a person must calculate the usable volume of a dam to determine whether that dam will be able to providing water for the area during dry periods. The shape of the dam will impact the volume of the dam. For example, a dam with steep sides can hold as much water as a dam with gentle slopes of the same length and area of the top of the dam.
Total Water and Usable Water in a Dam
However, the slope of the sides of a dam can impact how many livestock can approach the dam, as well as the stability of the bank of the dam. To calculate the total volume of the dam, a person must know the length of the dam’s top, the width of the dam at the top, and the average depth of the water in the dam. These three measurements can be use to calculate the area of the dam using the area of a trapezoid, where the area will be in acre or megaliters.
This calculation of the area of the dam is the gross capacity of the dam, which does not account for any water that may be lost to sediment in the dam or to the portion of the dam that lie beneath the pump intake of the dam. In addition to calculating the gross capacity of the dam, the person must also account for the freeboard and unusable reserve of the dam. Freeboard is the area between the water in the dam and the top of the dam; this provide space for the dam to fill with water, but also helps to ensure that the dam does not flood due to rainfall or the resulting formation of waves.
The unusable reserve of the dam is the volume of water that the dam cannot utilize for a variety of reasons, such as if the water is too shallow, too muddy, or replaced by sediment filling the dam. Both of these variables will reduce the total and usable storage capacity of the dam. The calculator that is provided reduces the gross capacity of the dam to account for these percentage to provide the usable storage of the dam.
The size of the area that may refill the dam and the coefficient of the area that replenishes the dam will impact how quickly the dam can refill with water. The area that replenishes the dam with water is referred to as the catchment area of the dam. Each area has a different coefficient for how much of the precipitation falls into the catchment area of the dam.
For instance, a sandy pasture will have a low coefficient for rainfall that will run into the dam, whereas a compacted laneway may have a high coefficient of how much of the rain will run into the dam. Thus, if the amount of runoff from the land that replenishes the dam is less than the amount of water that is evaporating from the dam or being used by the livestock, the dam will eventual dry up. A dam can continuously lose water to the process of evaporation.
The dam will lose water to evaporation each year, even if there are no irrigation effort occurring within the dam. In order to calculate the amount of evaporation, the pan data for the area can be used to determine the amount of evaporation from the dam. Additionally, the person can separate the stock demand of the area versus the irrigation demand for the dam calculator.
Because stock demand occurs every day at the dam, but irrigation demand may not occur each day, separating these two demands allow for the calculation of the reserve-days that the dam will have for the livestock. The dam calculator will provide several linked result to the dam owner or planner. For instance, the calculator can provide the total capacity of the dam, the usable storage of the dam, the annual net balance of the dam, and the reserve-days that the dam will have.
The total capacity of the dam has already been discussed. The usable storage has been discussed. The annual net balance will provide information for the dam owner as to whether the dam will gain or lose water within the year.
A positive result will indicate that the dam will refill, but a negative result will indicate that the dam will lose water over time. The reserve-days that is calculated will indicate how many days the usable storage of the dam will last during dry periods. A low number for this result will indicate that either the size of the dam need to be increased or that the demand for water from the dam needs to be reduce.
Finally, these results can be used to create a decision for the dam planner as to whether or not the dam is adequate to supply the water requirement of the livestock that will reside within the area.
