The Yeo from Ilchester to Langport

Witcombe Bottom hydrology

Water entering the Witcombe Bottom Moor comes from the higher land to the south. It eventually flows into Witcombe Main Drain in Witcombe Bottom from where it is automatically pumped up into the Yeo at regular intervals. In normal operation, no water from the Yeo enters this Moor

The phosphate concentration in the Yeo has consistently remained around 0.4 to 0.5 ppm for the last year. The concentration of phosphate in the Main Drain at Witcombe Bottom is variable and dependent on the two sources. One source, however, Tintinhull STW, has a roughly constant outflow and hence the variability is likely due to Witcombe Dairy, a large enclosed unit producing seasonally variable concentrations of phosphate in several rhynes.

The rhynes further east of these two sources significantly show barely detectable phosphate.

Witcombe Bottom during the summer
The dry summer saw the volume of water pumped from Witcombe Main Drain into the Yeo fall to below 30minutes per day which equates to around 1000 m3/day (see below). This compares with 30minutes in around 6 hours during avarage rain periods.

Slowing the flow rate through the Drain allowed phosphate to be absorbed during the passage of the water along the drain creating an increasingly large difference in concentration between top and bottom during the dry May such that by May 15th no phosphate was detected at the pump compared with 1.6 ppm at the top end of the Drain.

By July there was little change in the low pump flow rate but between 200 and 300 heifers were habitually poaching the bottom end of the Drain which was their only water supply. A phpsphate concentration of 2.5 ppm was recorded in the now largely stagnant water (below).

Wetmoor hydrology

Water entering Wetmoor comes mainly from slightly raised agricultural land to the south. The water flows in a series of rhynes down three levels controlled by sluices. The overall drop is a little over a metre in 5km. It eventually accumulates in Long Load Main Drain and is raised into the Yeo at Huish Pumping Station.

Water is fed onto the Moor from the Yeo from time to time via sluices near Long Load to maintain the required level and also to refresh the Moor rhynes by creating some water movement. There is some flow into the Moor from the west (west of the Muchelney-Langport road) which is fed in summer from the Parrett near Muchelney Lakes. This collects in Thorney Moor Main Drain just south of the Yeo at Langport. This joins Long Load Main Drain just before the pumps.

The volume of water flowing onto the Moor in summer from the Yeo and Parrett is very small compared with the total flow in these two main rivers (no exact figures are available).

Long Load Main Drain showed barely detectable phosphate throughout its length when the inlets are closed (winter). When the inlets are opened water wih a high phosphate level is let into the moor. Testing shows that this concentration reduces as the water spreads out through the moor rhynes. This could be caused by dilution but the evidence from spring 2022 suggests that the excessive phosphate coming from the Yeo is removed by growing plants in the rhynes and possibly also by the sediment. This mechanism is commonly observed elsewhere, most obviously in the Stourhead study and the ponds of Martock

The Thorney Drain brings water from the south w around Muchelney which in summer includes water from an inlet from the Parrett. This drain was free of phosphate in May but by July showd a concentration of around 0.8ppm, by which time the concentration in the Parrett which was feeding the Moor was very high indeed at around 2.5 ppm.

Comments
1 The main rivers act as drains rather than water supplies for the Moors except for dry summer periods when it is fed from three inlets, two from the Yeo and one from the Parrett.

2 The main inflow comes from nearby SSSI farmland and free of excess phosphate

3 Opening the inlets from the Parrett and Yeo in summer allowed in water with a high level of phosphate (2.5 and 0.8 ppm respectively). This was reduced to around 0.8 and 0.2 respectively as it flowed through to the pumps.

4 Knowing the flow rates, such as the rate of flow out of Witcombe Main drain permits some highly significan calculations. For example, in May 22, the flow down Witcombe Bottom was slow enough to allow natural processes to absorb around 2kg of phosphate per day. The main souces of this were Tintinhull STW and Witcombe dairy. The significance of this is that this single process is of the same order as the Entrade offset target for the whole of the Yeo catchment (0.95 tonnes/year).