Technical/data FAQs

A drought is a period of below-average natural water availability, caused by climatic variability. 
An event will likely start off with a meteorological drought; long-term or extreme below average rainfall. This could happen at any time of year, including autumn and winter which is typically when water resources should be getting topped up. 
Over time this leads to a drop in water stores in lochs and groundwater, and results in a hydrological drought. This is when we see prolonged low-flows in rivers.
Agricultural drought is a term used which is specifically linked to reduced soil moisture levels impacting on crop growing. This can happen over shorter timescales, such as a few hot and dry weeks in the summer.
Water scarcity is when the water supply becomes insufficient for demand. Water scarcity intensifies if the quantity of water available decreases.   

Soil moisture is the water content of the soil and is the main route for plants to obtain moisture. Soil moisture deficit is how much water (in mm) is needed to bring the soil moisture back to the maximum amount of water the soil can hold.

Groundwater is simply water that lies below the surface of the ground. When it rains, some of the rainwater runs off directly into rivers and lochs, and some percolates down through pores in the soil to the water table, below which the ground is saturated with water.  Groundwater below the water table flows slowly through the ground, eventually draining into rivers, lochs and the sea.  It can also be drawn directly from the ground using wells or boreholes, or emerge naturally as a spring.  

You might have a mental image of underground pools in caves, and rivers flowing through big gaps in the rock, but in general, most groundwater flows through pores between grains (in a similar way to percolation through soil), and through cracks and fractures in the rock. This means that groundwater flows much more slowly than rivers on the surface.  

The water table is the boundary within the ground below which all pores and fractures are filled with water (fully saturated). The water table moves seasonally up and down depending on the amount of rainfall percolating through the soil. In winter months, when rainfall is greater, the water table is higher (closer to the surface). In the summer months when rainfall is less the water table is lower.

Groundwater is an important factor in water scarcity, because it responds more slowly to changes in rainfall.  The water table generally drops slowly over the summer, and then refills again over the winter.  

Drought does affect groundwater, but generally on a different timing to the effects on surface waters.  During dry periods, groundwater is usually slower to react than rivers.  This means it sometimes provides a more reliable supply, with less risk of environmental damage, than trying to take water from depleted rivers.  However, long-term droughts, can cause the water table to become slowly but steadily lower over time, even if the rivers above still rise and fall in response to short-term rainfall. This increases the risk that the water table becomes low enough in the summer that shallow wells, boreholes and springs start to dry up.  

Please refer to the SEPA Data publication page which includes information on how to access river flow, water levels and rainfall.