Soil moisture describes the amount of water contained in the soil. When rain falls it makes the upper layer of soil wet and this water slowly travels downwards into soil depth. How wet a soil layer becomes depends on two factors, firstly the period over which rain is falling, for example when a sudden down pour of rain happens most water runs over the surface into rivers, lakes, or reservoirs; a gentle steady rainfall event is more favourable to water penetrating into soil depth. Secondly the soil type is important, and grain size of soils is a determining factor for soil moisture. Fine grains, such as clay, have finer channels between grains and at this microscopic level water molecules are generally tightly associated with the clay particles. Clayey soils hold their water for longer than sandy soils and the percolation down to the groundwater is slower. Also, soils rich in organic matter such as humus, are able to hold 80 -90% of their weight in water, making such soil types good stores of water.

Moisture in the soil percolates down to recharge ground water and aquifer stores. In addition it is important for vegetation as the roots of the plants take up water from different depths in the soil. Soil moisture ranges from fully saturated (100%), where all spaces in the soil are filled with water to dry where no water is contained in the soil (0%).

Soil moisture is one of the variables generated by the JULES model and the modelled outputs are mapped here.
The JULES model, represent the land surface and soil moisture is calculated for different soil types and different rates of rainfall, as well as water loss through evaporation, surface runoff and percolation.