The Q&A: Todd Caldwell

With each issue, Trib+Water brings you an interview with experts on water-related issues. Here is this week's subject:

Todd Caldwell is a hydrologist and geoscientist at the Bureau of Economic Geology at the University of Texas at Austin. He specializes in field investigations and numerical modeling associated with soil and vadose zone processes and its application to remote sensing of water resources. His current research focuses on soil moisture monitoring, modeling and scaling, as well as soil-plant interactions, near-surface geophysics and evapotranspiration. Caldwell is currently the principal investigator at the bureau for NASA's Soil Moisture Active Passive satellite project, or SMAP, to measure and model soil moisture.

Editor's note: This interview has been edited for length and clarity.

Trib+Water: How did you get involved in this satellite project to measure the state's soil condition?

Todd Caldwell: Soil moisture is one piece of the whole water cycle, and it has kind of always been the overlooked one, which is why SMAP came about. The measurements weren’t there. There is not very much water in the ground right now to use for this data. Until now, everything has come out of a numerical model for the last decade or so, and there are a lot of flaws in that. We showed some flaws in a 2013 paper that showed in Texas during the 2011 drought, we lost somewhere around 62 cubic kilometers of water total, which is a ton.

Trib+Water: How much of that was lost to the soil?

Caldwell: We looked at land surface models of another satellite called GRACE that the Center for Space Research here at UT is involved in, these numerical models predictions of soil moisture, the top one meter. When we extracted all that data across Texas, anywhere between 25 and 100 percent of that loss could have come from that soil alone.

That makes a lot of sense because we saw groundwater drop across the state. Groundwater is really slow to respond to single-year droughts or even five-year drought. Generally, it is because people are pumping not necessarily environmental, climate-driven events; it is more of an anthropogenic response.

When we looked at the model, we saw that we could have lost a lot of it to the soil. But those uncertainties were really high, anywhere from 25 percent of that water to 100 percent, and we don’t have a good way to decrease that uncertainty unless we have real data to look at which models are corresponding best to the real data. That got us going in soil moisture, in setting up some monitoring efforts.

We really wanted to start evolving a monitoring network. When the SMAP mission came along in 2012, the drought has already happened. We were worried that the soil moisture is completely depleted across the state, and we have to refill that soil moisture before there is enough excess water to get to the reservoirs and into the groundwater. If the deficit is really this big, it would be about 3.5 inches of water that would have to cover the entire state of Texas to refill all that water.

That led us to asking what data is in Texas. There isn’t much. We had communications with the jet propulsion laboratory that was building the SMAP satellite. NASA is very good at building things for space, but their specialty is not here on the ground, and that is where we came in. They saw us as a good partner and said they were looking for a validation site to give a better look into the water cycle and particularly soil moisture. We started developing the idea of putting the sensors in the Hill Country.

Through this detailed monitoring, we can downscale the larger satellite footprint to field-scale level that can be used by the water development board or the underground water conservation districts that run these areas to determine who is more severely hit, when the drought is actually over, when we can expect better or worse yields from crops, where we can expect more biomass available for foraging cattle, fire and flood forecasting.

Trib+Water: The SMAP satellite launched Saturday. Has everything gone according to plan?

Caldwell: It is a slow process. When it launched, the first thing it needs to do is separate from the booster rocket, which is a feat in itself. Two hours in, it deployed its solar panel. That was the next hurdle. Now they can communicate with it. It will sit there and acclimate to the space environment for about two weeks before it begins to deploy a very large antenna, like an umbrella.

Trib+Water: At what point will it be providing you with data?

Caldwell: It still has a few periods in the mission to go. It will start spinning, and once the inertia evens out, it will start collecting real background, baseline data, for about six weeks. That period involves checking the sensor calibrations, the systems and geo-positioning.

In about two months, it will start collecting the passive radiometer measurements and the active microwave measurements. That is when it will start its science mission. The beginning few months are the calibration period. They don’t have any real data to calibrate this with information from the land surface: crop heights, land topography, radio frequency interference from cellular towers and cities. All this stuff gets ingested after it goes around the Earth a few times. It starts to pull this together and come up with soil moisture estimates.

After the calibration period, there is a six-month validation period where they just let it run. It is collecting data then, and here at UT, we will start getting the real science data and start being able to see what it looks like and see how we can use its application from the flood prediction to the drought monitoring.     

Trib+Water: How can this data inform water policy? 

Caldwell: There a couple of ideal ways it can inform policy. It will aid in determining what irrigation demands might be in our more irrigated sections. We’ll know what the baseline soil moisture is in the spring before planting happens. It will also serve as a background rain gauge for places that we don’t have available data to know how moist the ground is. We never have a good handle on how much water irrigators are going to need.

We generally don’t know how well the soil is holding the moisture or how much it has rained. It will give us a better handle on some of the statewide water needs that various areas are going to have. It is not just agriculture, but also municipal. The rain eventually makes its way to our drinking water systems, and without having some of this data, it is more of guesswork. Right now, Texas is outside the bounds we are usually in. Our models have been calibrated within certain periods of rain, and now it hasn’t rained in so long that most of our forecasting models are troubled.