Soil can regulate the drainage, flow and storage of water and solutes, which includes nitrogen, phosphorus, pesticides, and other nutrients and compounds dissolved in the water. With proper functioning, soil partitions water for groundwater recharge and use by plants and animals.
Soil water relations can be assessed by measuring or observing the following indicators:
Salinity and Sodicity Indicators including electrical conductivity, exchangeable sodium percentage, sodium, and sodium absorption ratio
When rain or irrigation water falls to earth, some of the water will infiltrate into the soil and some will flow over the surface. If the soil is loose, porous, and has a stable structure, a drop of water will be likely to infiltrate. If the soil has few openings and unstable structure so that a crust forms and seals the soil surface, a drop of water will be more likely to run over the surface. Plants are also important in determining the fate of water. Leaves intercept water so some evaporates before it ever reaches the soil, and leaves and plant residue protect the soil so rain hits more gently. Roots and residue slow down the flow of water over land so water has more time to soak in.
If the soil becomes saturated, some water will drain down to groundwater. The remainder will be held in the soil until it evaporates or is drawn into plant roots, eventually transpiring from leaves. At all these stages water is carrying sediment, organic matter, plant nutrients such as nitrogen and phosphorus, pesticides, and other dissolved or suspended compounds. Water flowing over the surface may carry sediment and nutrients into lakes. Water draining into groundwater may contain nitrate or pesticides. Where does rainwater go after it falls on your property? During a downpour, watch where it flows and where it ponds. After the rain, notice how the soil surface dries more slowly under residue or mulch compared to bare soil.