Is the drought over.......?

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  • Figure 1. This bar chart shows the difference in monthly rainfall compared to the 1971-2000 average for the past 2 years. 17 of the last 24 months have experienced below average rainfall.

    Figure 2. A map indicating the areas of the UK currently affected by the drought (shaded in orange).

    Figure 3. A graph indicating rainfall totals so far this month at Norwich. A change to more unsettled conditions this month has lead to above average rainfall.

    Figure 4. 24 hour rainfall totals up to 6GMT on the 19th April 2012. Low pressure centered over the UK brought heavy showers or longer spells of rain across the UK. However, with the nature of showers, some areas received much more rain than others.

  • Is the drought over.......?
    21.04.2012 14:22

     

    .....No! After two dry winters, southern and central England has been declared as a drought affected area. This led 7 water companies in the south-east of England to introduce a hosepipe ban from the 5th April, thought to affect around 20 million people, or around a third of the UK population. However, since these measures were put into place, it seems like much of the country has been lashed by bands of rain, showers and even thunderstorms. But, as I will now explain, although there has been some much needed rainfall over the last few weeks, we are still along way off from alleviating the drought. 

    Figure 1 shows the difference in monthly rainfall compared to the 1971-2000 average over the last 2 years in the southern half of England. 17 out of the last 24 months have had below average rainfall, with 6 of these experiencing less than half of the usual precipitation for that month. Only 2 months out of the last 2 years have been significantly wetter than average (above 120%). By the end of March 2012, many rivers across much of England and Wales were reporting exceptionally low flows, with some rivers such as the Medway in Kent and the Stour in Dorset experiencing record low flows. Many groundwater sources (from which many people in southern England receive their water from) were also found to have exceptionally low levels or even record low levels. Therefore, as indicated by figure 2, a huge area of England currently has a significant rainfall deficit and has been declared as drought affected.

    However, after a dry and warm March, the UK has been dominated by low pressure systems since the start of the month, bringing bands of rain and April showers to much of the country. An example of this can be seen in figure 3, which shows that Norwich has received well above its usual April rainfall already (65.6mm), with over a week to go until the end of the month. In fact, 145% of the 1971-200 average rainfall has been recorded in the city so far.  

    The recent rainfall has obviously been very helpful in the short-term, allowing surface soil moisture deficits to be reduced and giving much needed water to farmlands, wetlands and rivers. However, we are a long way off until the drought is over, with these showers only being able to replenish just a small amount of the moisture deficit which has built up over the last 2 years. Also, with the nature of showers, areas reasonably close together can experience significant differences in the amount of rainfall they receive (see figure 4).

    Steady winter rainfall is the usual driver for the replenishment of the longer term sources of water, such as groundwater and reservoirs. These sources are generally used in the summer months when river levels tend to reduce. Moderate and persistent rainfall (more usual in the winter) allows moisture to continually filter through soil and rocks until it is able to become stored in groundwater aquifers in permeable rocks such as chalk, limestone and sandstone. Once we move into the summer half of the year (i.e. mid March to mid September), there are several reasons why this flow of water can be restricted. Rainfall in the summer tends to be in the form of short and intense showers, preventing the rainfall to filter into the ground. Vegetation is obviously more abundant in the summer and plants require substantial quantities of moisture and this combined with higher temperatures leads to increased evaporation from the ground and vegetation reducing the amount of moisture stored in the ground. 

    Much of the rainfall we have received over the last couple of weeks has been in the form of heavy downpours and thunderstorms. Large quantities of water have fallen to the surface in a very short space of time, preventing much of this water from infiltrating into the soil. This therefore has meant much of the water has run over the surface, emptying into drains or rivers. Although this has allowed stream and river flows to improve, this therefore prevents the replenishment of groundwater sources. 

    The forecast for the next week is that the weather will remain unsettled with further heavy showers or longer spells of rain. Although this will continue to help the on-going drought conditions, many more months of above average rainfall will be required before water supplies return to normal.

    By: Chris Burton