Depression Storage and Evaporation Guide
Why Depression Storage Matters
Depression storage is the water held in small surface pockets. It appears before clear surface runoff begins. Pavement joints, soil hollows, tire ruts, grass thatch, and rough concrete can all trap water. The stored depth may look small, yet the volume becomes important when the drainage area is large.
Evaporation in the Balance
Evaporation removes water from these pockets during and after a storm. The loss depends on evaporation rate, time, exposed area, and an adjustment coefficient. The coefficient lets the calculator represent shade, wind, surface cover, and water availability. A value near one means the entered rate is fully active.
How the Equation Works
The calculator first converts every depth and area to metric base units. It multiplies contributing area by depression depth to estimate maximum storage. It then adds rainfall volume and optional inflow. Initial water already stored is counted too. Evaporation volume is subtracted from available water. Filled storage is limited by the storage capacity. Any remaining water becomes potential runoff volume.
Practical Uses
This tool helps compare early storm losses, basin wetting behavior, and small site storage. It is useful for drainage sketches, stormwater checks, parking lot estimates, and classroom hydrology work. Designers can test how rougher surfaces delay runoff. Students can see how depth units, area units, and duration change the final volume.
Reading the Results
Use the storage filled result to see how much pocket volume is occupied. Use the evaporation loss result to see the water removed by weather conditions. Use the remaining runoff depth to compare with rainfall depth. A zero runoff result does not mean no rain fell. It means storage and evaporation absorbed the available water under the selected inputs.
Good Input Practice
Choose realistic storage depths for the surface type. Smooth pavement may store little water. Turf or uneven soil may store more. Enter measured evaporation rates when possible. For planning checks, test low, expected, and high cases. This gives a safer range for drainage decisions.
Limitations
The method is a planning estimate, not a complete hydrologic model. It does not replace field surveys, local design standards, or calibrated runoff software. Always review slopes, inlets, soil sealing, maintenance, access, and site safety before final decisions.