Waterwise, September 2011
The ‘Hydrological New Year’ occurs on 1 October (in the Northern Hemisphere) and it’s the point when the hydrological cycle is in balance. After 1 October rainfall starts to fill up the water reserves in the ground until 1 April (Middlepoint of the Hydrological Year) when evaporation starts to deplete the water stored in the ground, this carries on until 1 October when it starts to replenish, and the cycle begins again.
In the Southern Hemisphere it’s the other way around (though obviously this is all based on averages). We don’t often talk about the 1 April midpoint as people would think it was some sort of April Fools’ prank. But it’s not – in fact, the origin of April Fools’ jokes was due to the reality that in many ancient cultures the first of April was the start of the year – and this was probably linked to their knowledge of hydrological cycles.
It might seem surprising that, after a summer with an average daily temperature of 13 degrees and loads of rain, 1 October essentially represents the start of the rainy season. But it’s true – and it’s actually good news: this rainy season is essential for securing water supplies. In the north and west of Great Britain water supplies mainly come from rivers and reservoirs, the rivers in this part of the country are fairly fast flowing and respond quite quickly to rainfall, therefore long-term water resources rely on full reservoirs which are mainly fed by winter rain.
Reservoirs such as Bala and Vernwy which supply the conurbations of the Midlands and the North West are fed from the rainfall on the Welsh mountains. The situation is different in the south and east where water supplies are drawn from groundwater as well as rivers and reservoirs. Groundwater is the water stored in the chalk, limestone and sandstone aquifers. These aquifers are filled slowly by infiltrating rain; winter rainfall is essential for recharge. In 2005 and 2006 there was low winter rainfall which led to the lowest ever recorded groundwater levels in parts of southern England and resulted in widespread drought.
Unfortunately, summer rainfall contributes very little to aquifer recharge as evapotranspiration (direct loss from the soil and via growing plants) is high and even heavy rainfall can just run off the dry surface of the soil without infiltrating into the aquifer. This can lead to the phenomenon of droughts and floods occurring at the same time which we experienced in parts of England in 2006 and is quite common in Australia.
The management of water resources is a lot more challenging than it appears and changing rainfall patterns makes it more difficult. In particular the increase in intensity of rainfall means more erratic recharge of groundwater and filling of reservoirs, and more localized rainfall increases the chance of the rain missing the catchment for the reservoirs. This is already becoming more common in the northern part of the British Isles where catchments are small and steep.
Groundwater recharge is the hydrological process by which water moves down through the ground from surface water into groundwater, which is the water held in underground rocks or aquifers such as the chalk in Southern England. The rate at which water moves down through the soil depends on the dryness of the soil and the amount of water taken up by plant growth. Generally in the summer rainfall is lost by evaporation as it falls on the surface and then is taken up by plants and lost through transpiration before it gets a chance to move through the soil to recharge the aquifer. In the winter there is less evaporation and the rain can move more quickly through the wetter soil and there is less plant growth so less transpiration – therefore more of the rain reaches the aquifer.
We are now at that point in the calendar where we switch from a time of loss to a time of recharge. As the days get shorter, and the weather wetter, you might feel that things are winding down. But remember, it’s not the end of the year: in hydrological terms, it’s a brand new year.
Waterwise is a UK NGO focused on decreasing water consumption in the UK and building the evidence base for large scale water efficiency. Waterwise is the leading authority on water efficiency in the UK.
