Evapotranspiration rate, or the amount of water that can be evaporated or transpired from a surface over a specified duration, varies by latitude, topography, altitude, wind conditions, and time of year, as well as surface characteristics such as permeability, albedo, and types of vegetation.
In some climates, evapotranspiration varies widely through the seasons; green infrastructure should be designed with consideration for high and low expected conditions. Irrigation may be necessary during the very dry season, while fast infiltration is desirable during the very rainy seasons. In cold climates, snow storage is critical during winter.
Green infrastructure elements and systems may be designed to infiltrate a specified amount of stormwater; for example, a system goal may be to infiltrate the first inch of precipitation of each storm event in a tributary area, or to match pre-development runoff conditions.
Native underlying soils affect the infiltration rate. Regions with naturally fast-draining soils, especially gravel or sandy soils, may have dramatic infiltration capacity, but require accounting for water seepage around built structures. Potential effects on local drinking water supplies may also need to be considered in areas with high infiltration capacity. In areas with shallow bedrock or clay, infiltration rates may be naturally low. A high groundwater table may also affect a city’s ability to implement green stormwater infrastructure.
Plants and trees absorb stormwater runoff and stabilize the soil against erosion. GSI projects should use native, non-invasive, drought-tolerant vegetation. Select plants that can tolerate the inundation of stormwater runoff but also be adaptable to local climate conditions such as the dry season or severe cold conditions. In snowy climates, select plants that can tolerate salt.