Soil trench in the La Mesa surface, near Socorro New Mexico, revealing "calcic pipes". White areas are solid calcium carbonate precipitates, whereas darker areas represent "pipes" were there are no precipitates and there is preferential infiltration into the subsurface.

Investigators
Jan Hendrickx
Eric Small

Graduate Students
Sung-ho Hong
Julie Woolslayer

Collaborators

• Wim Bastiaanssen, International Institute for Aerospace Survey and Earth Sciences, The Netherlands
• Brian Borchers, Department of Mathematics, New Mexico Tech
• Bruce Harrison, Department of Earth and Environmental Science, New Mexico Tech
  
• Paul Neville, Earth Data Analysis Center, University of New Mexico
  
• James Cleverly, Department of Biology, University of New Mexico
  
• Rob Bowman, Department of Earth and Environmental Science, New Mexico Tech

Information on regional distributions of soil moisture and evapotranspiration in river basins is critical for the prediction of plant water availability, land surface evaporation and transpiration, runoff generation, contaminant movement through soils, ground water recharge, and irrigation scheduling. In this project we are implementing and further developing the Surface Energy Balance Algorithm for Land (SEBAL) for the Rio Grande Basin. This algorithm uses remotely sensed optical imagery for the determination of the regional distribution of the energy balance components and soil moisture. We have verified that SEBAL yields reasonable estimates for daily evapotranspiration (ET) in irrigated fields, riparian areas, and on arid ranch lands in the Middle Rio Grande Basin. Results for mountain slopes facing west and basalt flows are questionable and need improvement. Our regional distributions of ET and soil moisture are essential as input and validation data for the Los Alamos Rio Grande Basin hydrological model. In addition, our ET images will be available for other SAHRA researchers and stakeholders (such as the MRGCD - Middle Rio Grande Conservancy District) who have an interest in ET.

Activities and Results

In year one, we monitored soil water content in the top 5 cm of the soil at fourteen sites in the Sevilleta NSF - LTER site. We found a significant linear relationship between soil water content and the reflectance of selected wavelengths of the electromagnetic spectrum measured by LandSat. Thus, this highly empirical method may have potential for calibration of LandSat images to regional soil water content distributions. In year two, we validated SEBAL for conditions in the Rio Grande Basin. We analyzed three LandSat images using two versions of the SEBAL code: 1) surface roughness lengths derived from an empirical relationship with the Normalized Difference Vegetation Index (NDVI) (WaterWatch); and 2) surface roughness lengths derived from the more or less known height of vegetation and a land use classification from the LandSat image (NMT). SEBAL results were compared with eddy covariance measurements at sites run by Dr. James Cleverly (UNM) and Dr. Eric Small (NMT). During the past year we have focused all our efforts on the implementation of SEBAL on our computers at New Mexico Tech. We decided to not continue with the soil water content measurements in the top 5 cm of the soil in the Sevilleta. The data from the fourteen different sites were quite similar from dry day to dry day and from dry site to dry site. Once, we have a number of SEBAL generated soil moisture maps at different dates during the year we will use these data for determination of optimal locations for soil moisture monitoring and resume field measurements.

A comparison between the evapotranspiration rates measured in the field on September 14, 2000, in the Middle Rio Grande Basin and those derived with SEBAL is presented in Table 1. Overall, both versions of SEBAL seem to represent very well relative ET differences and compare well with measured values. The ET values in the riparian areas for cottonwood and saltcedar fall within the noise of the eddy covariance measurements and SEBAL. For desert environments the SEBAL/WaterWatch yields lower values than the field measurements while the SEBAL/NMT yields higher. The ET image (Figure 3) is quite instructive for understanding the regional ET distribution. For example, it clearly shows the high ET rates in the irrigated fields and riparian areas in the Rio Grande Valley versus the low ET rates in the surrounding deserts. The city of Albuquerque has much higher ET rates than the surrounding desert. In the Estancia basin, the round center pivot systems have a much higher ET than the surrounding dry lands. The basalt flow in the southeastern corner of the image has a high ET. Although part of this is due to a misrepresentation of the soil heat flux (in this SEBAL analysis the soil heat flux has been calculated in the same way for the entire image), it may inform us about a relatively higher ET on basalt flows after a precipitation event.

Plans

We now know how to implement SEBAL under the conditions of the Middle Rio Grande Basin and obtain reasonable ET estimates for irrigated fields and riparian areas. However, a much more rigorous test of SEBAL is needed to assess its results for ranchland, mountain areas, and basalt flows. We plan three research activities to better understand the potential and limitation of SEBAL for the basin wide determination of evapotranspiration.

1) Sensitivity Analysis of SEBAL. We need to conduct a sensitivity analysis of SEBAL's empirical relationships to find out which are critical for the Rio Grande Basin. Examples are: a) the relation between the thermal infrared surface emissivity and the Normalized Difference Vegetation Index (NDVI); b) the relation between emissivity of atmosphere and air temperature; c) the relation between soil heat flux and soil temperature, albedo, and NDVI; d) the relation between crop height and Leaf Area Index (LAI); and e) the relationship between crop height and surface roughness for momentum transport among others.

2) Comparison of field measurements with SEBAL values. How can we compare an ET measurement from eddy correlation stations with a footprint of up to several hundred meters with a SEBAL derived ET rate? To do so with more confidence we plan to install at least one scintillometer for direct measurement of the sensible heat flux over distances of up to 2.5 km. We expect to learn much from adding an independent measurement of the sensible heat flux to the current field measurements and SEBAL-derived components of the energy balance.

3) Increase temporal resolution. Our current implementation of SEBAL is based on LandSat, which has a good spatial resolution of 30×30 m but the temporal resolution is, at best, once every 16 days. MODIS images have a spatial resolution of 1000×1000 m (in the thermal infrared band) but a temporal resolution of 1 to 2 days. The ideal situation for water managers would be to have information available with a spatial resolution of 30×30 (which would cover most individual fields) at a temporal resolution of 1 to 2 days. Therefore, we will explore how we can combine the information from LandSat images and MODIS images. This is necessary to up-scale the use of SEBAL to entire river basins.

References on SEBAL

Allen, R.G., W.B.M. Bastiaanssen, M. Tasumi, and A. Morse. 2001. Evapotranspiration on the watershed scale using the SEBAL model and LandSat Images. Paper Number 01-2224, ASAE, Annual International Meeting, Sacramento, California, July 30-August 1, 2001.

Bastiaanssen, W.G.M. 1995. Regionalization of surface flux densities and moisture indicators in composite terrain. Ph.D. Thesis, Wageningen Agricultural University. Appeared also as Report 109, DLO Winand Staring Centre, Wageningen, The Netherlands. 273 pp.

Bastiaanssen, W.G.M., H. Pelgrum, P. Droogers, H.A.R. de Bruin and M. Menenti. 1997. Area-average estimates of evaporation, wetness indicators and top soil moisture during two golden days in EFEDA. Agric. For. Meteor. 87:119-137.

Bastiaanssen, W.G.M., M. Menenti, R.A. Feddes, and A.A. M. Holtslag. 1998. A remote sensing surface energy balance algortithm for land (SEBAL). Part 1: Formulation. J. of Hydrology 198-212.

Bastiaanssen, W.G.M., H. Pelgrum, J. Wang, Y. Ma, J.F. Moreno, G.J. Roerink, R.A. Roebeling, and T. van der Wal. 1998. A remote sensing surface energy balance algortithm for land (SEBAL). Part 2: Validation. J. of Hydrology 212-213:213-229.

Van den Hurk, B.J.J.M., W.G.M. Bastiaanssen, H. Pelgrum, and E. van Meijgaard. 1997. A new methodology for assimilation of initial soil moisture fields in weather prediction models using Meteosat and NOAA data. J. of Applied Meteorology 36:1271-1283.