L is for Lag time

For the letter H, I posted about the hydrograph – a deceptively simple plot that represents the complex integration of the many processes that occur across a watershed to generate streamflow. Lag times are a key feature of hydrographs that help us better understand how a watershed functions. But let’s not limit ourselves to just this narrow definition.

The plot above shows an idealized storm hydrograph for a generic watershed. It even conveniently labels the lag – the time between peak precipitation (the grey bars poking down from the top) and peak streamflow (highest value on the curve). The greater the lag time, the more likely that the watershed is delaying and/or storing water somewhere across its diverse landscape. What types of things can delay water? Well, snowpack for one. If you have rain falling on snow, the rain can fill the pore spaces in the snow and not be released to the stream until they’re full. Another storage location is in the soil. If you have a really dry soil in the fall, the winter snowpack will accumulate on top of it – and when that pack melts, the water will first fill up the soil moisture before going to streamflow. Groundwater is a classic storage location. Water from snowmelt can go directly to groundwater recharge, and as a result there will be a long lag between peak precip and peak streamflow. Surface depressions can also be important. In something like a prairie pothole landscape, water will fill ponds – and lead to ponds connecting as they ‘fill and spill‘ – before streamflow is generated.

In other studies, however, we’re not looking directly at streamflow. We may instead be interested in, for example, the relationship between stream and air temperature. There’s been a lot of high profile work lately showing that the two are related, and that there’s a lag between peak air temperature and peak water temperature. This is usually a function of the time it takes a specific volume of water to warm by conduction from the air – and potentially from the stream substrate, which can also warm during the day. One of my students has been working on this topic and found that – in groundwater dominated systems – the lag time between air and stream temperature is super long, suggesting that they’re not directly related. The lag time is actually telling us that there’s something else driving stream temperature, and in our case it’s the contribution of spring snowmelt to groundwater recharge, and the subsequent release of that groundwater to the stream.
Lag time can also refer to the time between when you read something – and when you actually understand it.   I’m sure you have many personal examples of this from your everyday lives…