Combining fixed and mobile cosmic-ray neutron probes for realtime multiscale monitoring of soil moisture

Franz TE, School of Natural Resources, University of Nebraska-Lincoln, USA

1. IntroductionApproximately 40% of global food production comes from irrigatedagriculture. With th e increasing demand fo r food even greater pressures willbe placed on water resourc es within these systems. In this work we aimedto characterize th e spatial and temporal patterns of soil moisture at variousscales by combining fixed and ro ving cosmic-ray neu tron prob es at threestudy sites across an East-West precipitation gradien t overtopping the HighPlains Aquifer (HPA). Each of th e study sites consisted of co ars e scalemapping of the enti re domain. By using a simplistic data merging techniquewe are able to produce a statistical daily soil moisture product at a vari ety ofkey spatial scales in support of irrigation water man agemen t technology: theindividual sprinkler (~102 m2) for variable rate irrigation, the individual pieslice (~103 m2) for variable speed irrigation, and the quarter sec tion (0.64km2) for uniform rate irrigation. In addition, we are able to provide a dailysoil moisture product over the whole study area at a vari ety of key remotesensing and modeling scal es 1, 9, and 144 km2. These products c an be us edto support SMAP/SMOS th rough calibration, validation, and value additionby statistical downscaling.

2. Study Site

Platte River Basin Cosmic-ray Monitoring Network ( 10fixed probes)

SiteResearchObjectives

1. Design of cos t effec tive, accurate, and pragm aticmonitoring techniques in support of variablerate and variable speed sprinkler irrigationpackages.

2. Design of monitoring technique to providespatial es timates of top boundary condition(recharge) in support of groundwater flowmodel at study site.

3. Design of basin wide monito ring of 100’s ofcenter pivots in support of scheduling irrigationtiming and depth for uniform rate irrigation.

3 1 2

3. Data Merging Technique

Fixed cosmic-ray neutron probemeasuring neutron counts everyhour

Roving cosmic-ray neutron probemeasuring neutron counts everyminute

1. Convert fi xed neutron counts int odaily averaged soil water content

2. Convert roving neutron count s intospatial soil water content map usi nggeostatistical methods

3. For speci fied geometry, per formlinear regression between areaaverage soil water content (rover )and corresponding (i n time ) fixedprobe soil water content

4. Use regression relationships,regression coefficients, and timeseries of fixed probe soil watercontent to construct a continuousdaily spatial product

Details of methodology can be found inFranz et al. (2015 GRL)

4. Precision Agriculture

Data merging results in support of variable rate irrigation

Data merging results in support of variable speed irrigation

5. Small Watershed Monitoring

Data merging results in support of Modflow grid

6. Watershed Monitoring

Data merging results in support of uniform rate irrigation

9. AcknowledgmentsThis research is supported bytheDaugherty Water for FoodInstitute atthe Univers ityofNebraska,ColdRegions Research Engineering Laboratory, USGS104B,and theUNLLaymanAward. Wewouldalsoliketothank TheNature Conservancy,Paulman Farms, NebraskaWaterBalanceAlliance, TheWestern IrrigationProject, and thePlatte River Recovery Project for providing access tofields ites ,on-s itesupport, andancillarydata. Wewouldliketothank GaryWomackandDarin Des ilets for support with the rover.Formore information about DWFI andthe Hydrogeophys ics labgroup pleasevis ithttp:/ / tfranz2.wix.com/trenton-franz

7. Summary and Conclusions1. Combining fixed and roving cosmic-ray neutron probes is a

feasible and cost effective strategy to continuously monitornear surface soil water content at a variety of scales.

2. Arbitrary geometry of the method makes it applicable towater resource management in aiding decision making inirrigation.

3. The arbitrary geometry makes the method applicable tomatching modeling grids or comparing to remote sensingproducts.

4. Accuracy of the method (R2 and p value) is dependent onselected spatiotemporal scale and rover mapping resolution.Daily products at ~50x50 m is feasible.

5. Data mining techniques ofneural networks and deep learningwill be explored to maximize correlation and flexibility withscaling.

8. ReferenceFranz, T. E., T. Wang, W. Av ery, C. Finkenbin er and L. Brocca (2015). "Combinedanalys is of soil moisture m easurem ents from roving and fixed cosmic ray neutronprobes for multiscale real-time monitoring." Geophysical Research Letters 42.