Where does all the water go!

In recent years, we seem to have been experiencing more downpours than gentle rain showers. So what happens to the water in a garden during such a deluge?

Water percolates into the ground at varying rates depending on the nature of the soil. This is a complex process but for average garden soil, not too much sand and not too much clay, water soaks in at about 4.8 mm/hour. Some water can accumulate on the surface but beyond another millimeter or two, it begins to run off. This runoff is moderated by plants when they are of sufficient size. However, as the plants grow, other factors influence efficacy of the rainfall.

Water in the garden is consumed by two processes: evaporation and transpiration. The combined processes are refereed to as evapotranspiration (EVP). Evaporation takes place from the soil surface and is strongly influenced by wind and sun. Transpiration takes place from leaf surfaces as plants breath and tends to be in the 5-8 mm per day range depending on zone. After a rainfall, evaporation starts off at a rate similar to transpiration but decreases as the soil dries out. Average EVP tends to be in the 5 mm range. Hence, one often hears that gardens require an inch of rain every 5 days (25 mm ~ 1 inch).

If a rainfall of 25 mm occurs in one hour for average soil on more or less level ground, approximately 5-8 mm might be absorbed and the rest drains away. As the leaf structure of plants expands, rain intercepted by leaves evaporates before reaching the soil surface, further reducing the amount of water provided to roots.

In a Seed Haven garden, a major portion of the rainfall will be captured and allowed to soak into the plant root zone regardless of the duration of the rain event. This is like having a mini rain barrel for each planting. The closer Seed Havens are placed together, the more complete the utilization of the rainwater. As well as slowing the flow of surface drainage, wind losses are reduced. Less watering will be required and plants will receive a healthy dose of nitrogen and oxygen.