How does water that falls on land get to streams?

Posted on April 8, 2020 by Anne Jefferson

It’s a rainy day and you can see that the water level is rising in your local stream. That’s because of the rain falling on the stream channel and its tributaries, right? Wrong.

In most watersheds, <1% of the land area is covered by streams, lakes, and wetlands, and that means that more than 99% of rain lands on vegetation or land rather than on water.  So in order to explain why water levels in streams and rivers rise during storms or shortly thereafter, we have to get the water from land downhill to the stream network. We call the processes that do that work streamflow generation mechanisms or runoff generation mechanisms. 

There are four classic streamflow generation mechanisms and I’ve cobbled together videos on each of them. If you’ve ever wanted to know more about what is happening under your feet in wet weather, and you’ve got about an hour to spare, you can learn all about it by working your way through the videos below. 

If videos aren’t your thing, or you want to supplement what you watch with some informative web pages and even practice quizzes, check out this resource from the COMET program.

Start here to learn about streamflow generation
If you’ve only got time for one video, this one is the content-rich introduction that you are looking for:

Terminology note: In the video above and in other resources listed below, you may see the phrase “runoff generation” used. I prefer the term “streamflow generation” because I’m interested in all of the ways that water gets to streams (or other water bodies), not just ways that involve surface runoff or overland flow. I also use the term streamflow generation to talk about how we get baseflow (i.e., water in the stream between storms), which is not runoff. 

More about infiltration-excess (or Hortonian) overland flow

I’ve put together the video below to tell you more about what happens when it rains too hard for a soil to infiltrate all of the water.

Thanks to Todd Walter of Cornell University for the illustrations in the middle of the video. To play with those some more or learn about the history of our understanding of infiltration excess overland flow, you should visit his website.

More about subsurface stormflow
In humid, vegetated areas, we often have low enough rain intensity and high enough infiltration capacity for water to infiltrate rather than run off overland. But even though the water is in the soil, it can still cause an increase in streamflow. I made a video to explain how.

More about saturation overland flow (and the variable source area concept)
Even if rain intensity is low and infiltration capacity is high, if it rains long enough things get really wet. Sometimes water runs off overland because the ground is saturated. The wetter things become, more areas become saturated. I try to explain in the video below.d

Thanks again to Todd Walter at Cornell for the excellent illustrations. To learn more about saturation overland flow and the related concept of variable source area, you should visit at Todd Walter’s page on saturation overland flow and variable source area to review the animated sequences (and much more).

Taking it slow: More about groundwater and its connection to streams
OK, so water can get to streams during and shortly after rain storms. But where does water in streams come from when it’s not raining? Or when it hasn’t rained in a long time? The answer is groundwater, and this video does a great job of talking us through some really basic concepts, including connections between groundwater, streams, and lakes and the potential effects of pumping groundwater.

Just can’t get enough? Master class on streamflow generation

If you think that streamflow generation mechanisms are just about the coolest thing that you’ve ever heard of, then you can learn so much more about them from one of the world’s leading experts on hillslope hydrology: Jeff McDonnell in this 3 hour short course. When I was a graduate student I got to take a 10 week hillslope hydrology class with Dr. McDonnell and it changed the way I think about the world beneath my feet. Listening to 3 hr recorded presentation isn’t nearly as good, but you could still learn a lot if you want to do a deep dive. 

Still can’t get enough? You can watch videos from McDonnell’s 10 week course. But as you do that, you should consider graduate school in hydrology, because you have clearly got the bug. I bet you get excited by mud, snowmelt, floods, and good, heavy rain storms too.

Categories: by Anne, hydrology

Measuring infiltration capacity in the field

Posted on March 15, 2020 by Anne Jefferson

Infiltration is the act of water moving from the land surface into the soil. Usually, we think about infiltration occurring in response to rainfall, where water is applied over the whole land surface. In that case, infiltration is important because it’s one of the deciding factors in how much rainfall becomes available to plants and groundwater, versus running off over the land surface and potentially causing erosion and flooding.

Infiltration is also important for applications like irrigation – where furrows or drips might apply water to one part of the surface and not others, in order to more efficiently provide water to crops. And infiltration is extremely important for stormwater green infrastructure, like rain gardens and bioretention cells, because we are explicitly designing them to be able to infiltrate water from impervious surfaces like parking lots and roof tops. For green infrastructure, the engineered soil properties are carefully designed to promote just the right amount of infiltration, but the performance of the green infrastructure can also be affected by the infiltration capacity of the surrounding “native” soils and sub-soil.

Based on the above, there are a lot of reasons hydrologists want to be able to measure infiltration, but what exactly are we going to measure? First we need some definitions. Typically, we talk about infiltration as a rate: how fast is water entering the soil? But let’s say it’s raining 0.5 cm/hr, sort of a drizzle. A lot of soils can soak that water in, no problem, giving an infiltration rate that is equal to rainfall rate. So, just talking about infiltration rate might not be helpful. Instead, we talk about infiltration capacity, the maximum rate at which water can enter a soil. But here’s the funny thing, infiltration capacity changes over the course of a rain storm (or irrigation event), so you can’t just measure infiltration capacity at any random point of time and assume you’ve captured a value that applies at other times.

Graph of infiltration rate (y-axis) vs. time (x-axis), curve decreases over time towards asymptote marked as ksat = 0.52 cm/hr

Infiltration rate in an Oklahoma sandy loam decreases over time, in a typical exponential fashion. The asymptote is the saturated hydraulic conductivity. [Image by Oklahoma State U., clink image for source.]

Fortunately, after a long enough period of time, the infiltration capacity starts to asymptotically approach a constant value. We call this steady-state rate the equilibrium infiltration capacity, which conveniently (and logically) is approximately equal to the the saturated hydraulic conductivity. And it’s that steady state rate that we most often want to measure. But how do we do it?

Cutaway view of two gray rings at land surface and a bulb of water below the soil surface, with a darker blue plume indicates water from the inner ring

Schematic of infiltration below a double ring infiltrometer. [Image by SDEC. Click image to go to source.]

The most common way to measure equilibrium infiltration capacity is through a double ring infiltrometer. These devices consist of two concentric rings (30 cm and 45-60 cm in diameter) pounded slightly into the soil and filled with water. The water from the outer ring helps wet the soil and infiltrates both vertically and laterally into the dry soil. The infiltration rate is measured in the inner ring , where infiltration and percolation are happening only vertically, thanks to the water from the outer ring doing the lateral movement. Water levels are maintained at the same depths in both rings. We say that outer ring reduces the boundary effects for the measurement of vertical infiltration in the inner ring. You can do infiltration capacity measurements with a single ring infiltrometer, but it’s considered less accurate because of these boundary effects.

Tests can be conducted in two ways: falling head and constant head. In a falling head test, water is added to the rings and the water level declines over time as infiltration occurs. In high infiltration capacity soils, it may be necessary to add water several times before steady state is achieved. Falling head tests require less equipment than constant head tests, but the math is a bit more complicated. In constant head tests, a device called a Mariotte bottle is added to the infiltrometer. A Mariotte bottle releases water so that a constant level (or head) is maintained inside the rings. This simplifies the math considerably.

The video below shows a falling head double ring infiltrometer test. [It’s probably not necessary to watch the whole thing, once you have a sense of how things are going.]

The next video is a fantastically informative USDA instructional video for how to set up and use a constant head double ring infiltrometer. [I personally enjoy watching this one slightly sped up.]

Can’t get enough double ring infiltrometer action? You can check out this video with step-by-step instructions for set up, with some sweet elevator music in the background.

As you are probably starting to realize, it takes a fair amount of work and quite a bit of water to make a double ring infiltrometer measurement. What happens if you need to get into a remote site or have a limited amount of water? Another device, called a Guelph permeameter, can help you out here. The Guelph permeameter works on a similar principle as the infiltrometer, but water coming out of it sets up a wet bulb of a specific known shape, from which you can calculate equilibrium infiltration capacity. With a Guelph permeameter, you make measurements at two different specified heads (usually 5 and 10 cm), and in each case allow infiltration to occur until a steady rate is reached. Then you plug your steady rates and your chosen head values into an equation to get a number for saturated hydraulic conductivity.

Here are some Watershed Hydrology students using a Guelph permeameter along the banks of a Pennsylvania stream. Note how much faster the water infiltrates at the 10 cm head (second half of the video) than the 5 cm head (first half of video). [Question for Watershed students: Can you figure out why?]

Another cool thing about Guelph permeameter is that you can fairly easily get vertical profiles of hydraulic conductivity or get the measurement at single specified depth below the surface (this would be useful for green infrastructure design, for example). The video below gives pretty detailed instructions for how to set up and use a Guelph permeameter and includes a demonstration of the capability to got deep.

Categories: by Anne, hydrology, teaching

How easily water moves in soil depends on how big the soil pores are, how well they are connected, and how wet they are

Posted on March 12, 2020 by Anne Jefferson

Hydraulic conductivity is the term used to describe how easily a fluid moves through a porous medium. In the case of water infiltrating into soils, the fluid is the water and the porous medium is the soil. We often use K to represent hydraulic conductivity when we are writing equations.

When all of the soil pores are full of water, we call this saturated hydraulic conductivity, Ksat. Saturated hydraulic conductivity is a function of how big and how well connected the soil pores are. In general, soils with lots of smaller particles (silts, clays) have smaller and less well connected pores than soils with lots of larger particles (like sands). Therefore, sandy soils have higher K than silty and clay-rich soils (even if they have less total pore space).

Circles showing sand grains and big pores and clay soil and small pores

Illustration of the different characteristics of pores relative to grain sizes

The difference in K can be several orders of magnitude:

Diagram showing the range of hydraulic conductivity values for different types of rocks and sediments

Image from: Heath, R.C., 1983. Basic ground-water hydrology, U.S. Geological Survey Water-Supply Paper 2220, 86p. (Public domain)

Across the full range of rocks and sediments, we can measure K over a range of 12 orders of magnitude (an order of magnitude is a factor of 10). Across a typical range of soils, K might vary by 4-5 orders of magnitude. That means that water can flow 10,000 times more easily in some soils than others.[Isn’t that mind-blowing?]

But in all soils, water moves more easily when all of the pores are water filled (Ksat) than when some pores have air in them (Kunsat). Why is that? This video explains…

Curious about the figure in the upper left part of the screen at the beginning of the video? Here’s a video that explains the soil characteristic curves for wetting and draining:

Categories: by Anne, hydrology

How wet is the unsaturated zone?

Posted on March 12, 2020 by Anne Jefferson

In my Watershed Hydrology class, we’ve been talking about soil moisture and water potential in relation to evaporation and infiltration. An important concept is the idea of the unsaturated zone, where the pores (or open spaces) in the soil, sediment or rock are not completely filled with water most of the time. In class, I went over a diagram of the unsaturated zone and talked about the capillary fringe. I found the figure below to be quite useful in summarizing the different parts of the unsaturated zone and how wet those parts usually are. Take a look at the figure now, and then I’ll define the relevant terms below the image.

Diagram of unsaturated zone showing root zone, intermediate zone, capillary fringe, groundwater, etc. usual range of water content and pressure state are also shown.
Figure 6-15 from Dingman, Physical Hydrology, 2nd Ed. Posted under fair use.

On the left, there is the usual range of water content, where water content is noted with θ (theta). All the way down, water content is always equal to or less than porosity (φ, phi). In the rooting zone, where both infiltration and evapotranspiration occur, water content is usually someplace above θpwp, where pwp stands for the wilting point. Below the rooting zone, there isn’t much effect from evapotranspiration, so water content tends to stay at or above θfc, the field capacity, which is when drainage due to gravity stops. Below the water table, you find the saturated zone, where water content is equal to porosity (φ, phi), by definition. Just above the saturated zone is the weird world of the capillary fringe. Here too water content is approximately equal to porosity, because water is drawn up into this zone by capillary effects (just like a sponge gets wet higher than level of the water when it is placed in a pool of water).

On the right side of the diagram, you can see the water potential, in this case expressed as pressure. All the way down to the water table, potential is negative and in the form of matric potential (ψ, psi). Remember, matric potential is always negative, and represents the tension being exerted on the water in the pores. At the water table, matric potential is zero and pressure potential is zero. Below the water table, matric potential goes away and pressure potential is positive, caused by the weight of the overlying water. One last point is that the top of the capillary fringe is where the matric potential, ψ, is equal to the air entry tension (ψae). That’s the point at which a significant amount air starts to occur in the pores, when then starts to change how easily water moves through them.

Want it drawn out for you again? The video below is a great explanation of the unsaturated zone (sometimes called the vadose zone) and related concepts.

Categories: by Anne, hydrology, teaching

Measuring evapotranspiration components

Posted on March 12, 2020 by Anne Jefferson

Evapotranspiration is often said to be the most difficult water balance component to directly measure. When water goes from liquid to vapor, you can’t exactly catch it in a bucket or measure flow in a channel. In my Watershed Hydrology class, we used a very simplified version of the weighing lysimeter to measure evapotranspiration from soils and potted plants. But how else can we measure evaporation and transpiration? Let’s start by talking about techniques used to measure evapotranspiration from particular surfaces (open water, soil, plant interiors (transpiration), and plant surfaces (interception)).

1. Open Water Evaporation

As we talked about in class, open water evaporation is first and foremost a function of the availability of energy (i.e., net radiation) at the water surface and the ease with which water vapor can mix into the atmosphere (i.e., humidity, wind). But properties of the water body also matter, including water depth, thermal stratification, size, heat content of inflows and outflows, vegetation (shading), turbidity and bottom reflectance (albedo), and salinity. Unfortunately, those variable water body properties make it a harder to get evaporation than it would be if it were solely energy, wind, and humidity.

If you want to measure lake evaporation, one approach is to measure all of the other hydrologic cycle components for the lake, including change in lake level for your change in storage. Then solve for evaporation, using the water balance. 

lake with arrows showing precipitation, evaporation, and surface and groundwater inflow and outflow

Example of the components needed for a lake water balance. This image is from the USGS, http://pubs.usgs.gov/wri/wrir-03-4238/ and is in the public domain.

If the water balance approach is not practical, you can measure the water balance of a very simplified system called an evaporation pan. Here’s how a standard (class A) evaporation pan works. The pan is 122 cm in diameter and 25 cm deep. It’s placed outside in an area free of obstructions and water level is maintained at 18-20 cm deep. The water is refilled as needed to maintain the desired level. Water level measurements are taken daily (giving evaporation as a depth) or the volume of water required to refill to the desired level is recorded (volume of added water divided by area of pan = evaporation as depth). Of course, since the pan is outside, it will also receive rainfall, so that needs to be measured and figured into the calculations:

Evaporation = beginning depth – ending depth (for days with no rain)

Evaporation = beginning depth – ending depth – rainfall depth (for days with rain)

Since a shallow pan evaporates more quickly than a deeper lake, to get lake evaporation you  multiply the pan evaporation rate from the pan by a lake coefficient (typically 0.7 to 0.75) that takes that into account.

Metal pan filed with water with a fence over the top, on a pallet in a mowed area.

A Bureau of Meteorology (Australia) class A evaporation pan, with an adjacent anemometer to measure wind speed. A rain gauge would also be located nearby. This file is licensed under the Creative Commons Attribution 3.0 Unported license from Wikimeida. Image by Bidgee.

2. Soil evaporation

The conceptually simplest way to measure soil evaporation is the weighing lysimeter, as long as your lysimeter doesn’t contain any vegetation. (The group who used the pots with soil and mulch was measuring soil evaporation in this approach.) The weighing lysimeter takes advantage of a mass balance approach to the water balance.

Another method takes advance of a heat balance. In this technique, fine scale measurements of soil temperature and thermal properties are used to calculate change in sensible heat storage at different depths from the soil surface. Over short distances and time scales, sensible heat changes as a function of latent heat (i.e., evaporation).

Measurements are made with carefully spaced thermocouples (i.e., thermometers). One of the thermocouples also has a heating element, that can send out pulses of heat. By measuring how those heat pulses are received at different distances, the latent heat can be calculated.

three pronged sensor with 3 thermocouples and a central heater. Variables are named in relevant positions.

Heat pulse sensor and associated measurements. Temperature (T), temperature gradient (dT/dz), volumetric heat capacity (C), and thermal conductivity (I) are the relevant variables. From Heitman et al., 2008, Water Resources Research.

You can read a lot more about the details of the method and math in Heitman et al., 2008, Water Resources Research, which is freely available. The diagram above shows how the apparatus works. Looking ahead, this approach is very similar to how sapflow in trees is measured to estimate transpiration.

3. Transpiration (evaporation from plant interiors)

Transpiration – or evaporation from the leaves and needles of vegetation – is really important in many landscapes. You’ve already experienced one approach to measuring it – our variation on the weighing lysimeter – with our potted plant experiments.

Measuring the change in weight of potted plants is a tried-and-true approach to estimating transpiration – especially if care is taken to reduce soil evaporation (think mulch or some sort of plastic sheeting). But there are limits on how big a plant you can put on a scale and larger weighing lysimeters can be very expensive to build and maintain. Even then, you don’t find many weighing lysimeters measuring full grown oak or fir trees! So other approaches are needed.

An approach that works with plants up to large shrubs or small trees is the tent method. Here the plant is enclosed in a clear plastic tent and the moisture content of the air entering and leaving the tent (via fans) is measured. When the moisture content of the air leaving the tent is higher than the air entering the tent, that’s because of evapotranspiration. And if the soil surface is covered, the difference in moisture is just due to transpiration.  BUT! It gets hot inside the tent so maintaining proper air flow is critical. Even so, the transpiration rates measured using the tent method might not be the same as would occur in the surrounding natural environment.

line drawing of leafy plant inside clear plastic tent with some sort of single level inlet and multi-level outlet structure

Illustration of the tent method for measuring evapotranspiration. Figure 4.5 in Brooks et al., 2012, Hydrology and the Management of Watersheds (your textbook).

A smaller, shorter-term variation on the tent method is porometry – which measures stomatal conductance. In porometry, a leaf is temporarily enclosed in a small chamber where changes in humidity are measured – and attributed to transpiration. Porometry is good at telling scientists how stomata are responding to environmental variations at really small scales, but isn’t good at providing larger scale data, because what the stomata are doing  can vary depending on the orientation of the leaves,  height in the tree, etc. Age of the plant, plant species, etc. also effect how stomata respond to environmental conditions – and that’s before you even get to the environmental variations themselves (temperature, humidity, wind, etc.). So you would need to make a LOT of stomatal conductance measurements via porometry to use this approach to accurately measure transpiration for large plants, like trees, for example. Nonetheless, it’s a pretty cool technique that can tell you a lot about how plants are taking up and releasing water. Here’s a video that explains more about how it works:

So if porometry, tenting, and weighing lysimeter style measurements are all limited to small plants, how the heck do you measure transpiration in larger trees? After all, in forested environments, they are going to be doing a lot of the transpiration we’re interested in. The answer is sap flow measurement, and here are two videos that explain what it is and how it works. The first video gives you a quick and simple explanation, while the second video is full of information and even shows example data!

4. Interception (evaporation from plant surfaces)

Interception is the precipitation that never makes it to the soil surface because it gets hung up on leaves and branches of vegetation or even the dead organic material on the forest floor (the litter). After the storm is over, this water evaporates before it ever reaches the ground. That process of being caught then evaporated is called interception. Interception can be a very important water balance term, in both the winter and summer. (I’ve written more about snowfall interception here.)

The simplest way to measure interception is to measure the precipitation in an open area and compare to precipitation measured under the forest canopy (through fall) and what runs down the stems of trees (stemflow). Interception is the difference. This ignores litter interception though and that’s a bit harder to measure. There are also some really cool new techniques demonstrated in the video below, that let researchers start to scale up from point measurements to a whole forest:

Categories: by Anne, hydrology, teaching