Importance of hydrology
The volume and movement of water acts as an important control on the temperature that water can attain. Large volumes of water are less responsive to changing heat inputs than smaller volumes. Consequently, the temperature of water lags behind solar inputs due to inertia. Water moves through river networks and, thus, the temperature of water at a site is partially determined by the conditions upstream. This inheritance of water from upstream means that it is important to acknowledge and manage whole river catchments rather than individual sites when considering the thermal regime. The inertia and inheritance of water in rivers leads to a complex and dynamic thermal regime and can make it difficult to identify the drivers and controls on water temperature.

Looking downstream towards Wolfescote Dale from Beresford
Groundwater springs
The hydrology of the River Dove is effected by substantial groundwater inputs associated with the limestone geology. Groundwater discharges into the river at a constant 8.4 degC and, consequently, has a substantial impact on the thermal regime. Where groundwater discharges all year around, the overall result is to dampen changes in temperature driven by solar radiation, including diurnal and seasonal cycles. In such areas, solar radiation is relatively insignificant to the thermal regime and, consequently, shade is of the channel is of negligible importance.  Other areas of the Dove have intermittent springs, where groundwater discharges for only some of the year. Changing weather affects groundwater supply and, therefore, the relative significance of solar radiation between years. For example, during the drought year of 2011 many springs dried up and summer water temperatures were high, whereas in 2012, which was one of the wettest on record, summer water temperatures were low despite air temperatures being similar.
Site D14, spring
Biggin Dale spring
Above left: Intermittent spring at Sprink. Above right: Intermittent spring flowing though Biggin Dale. Centre to far left: Three perennial springs flowing in Dovedale. 

Spring upstream of site D23
Spring emanating from a cave between site D23 and D24

A hysteresis loop of 15-minute maximum air temperature and water temperature measurements beginning at noon on the 11th July 2011.

Daily variations and hysteresis
Water temperature varies daily due to the diurnal cycle in solar radiation. We have quantified diurnal variations in temperature using hysteresis loops of air and water temperature (see graph to the left). Air temperature is on the x-axis and water temperature is on the y-axis. The loop covers 24-hours beginning at noon and each point is a 15-minute maximum temperature measurement. The open circles indicate daylight and the closed circles are night-time.

The hysteresis loop is formed because changes in water temperature lag behind those in air temperature. As a result, there are large periods of time where air and water temperature do not behave in parallel. River temperature generally reaches a maximum a few hours after air temperature. The shape of hysteresis loops is related to prevailing weather and hydrological conditions, in particularly the presence of groundwater. For more information see Wilby et al. (2014).

Thermal waves
Daily variations in temperature can be extreme. When large quantities of warm or cool water are suddenly inputted into the river, for example, due to summer rainstorms or snowmelt, a thermal wave is generated. The graph to the right shows a thermal wave propagating through the River Dove on the 28th June 2012 with temperature peaking at around 14:00 at site D1 and at 23:00 at site D16 and 01:00 at site D20. Although these events are rare in the LUTEN record (three in three years), they represent the most extreme rates of change, sometimes equivalent to 12°C per hour. For more information see Wilby and Johnson (in review).