SALSA collaborators from Mexico, France, and the USA will conduct an intensive field campaign to measure upland water and energy fluxes during August and September 1998. The SALSA "Mexico Campaign" will focus on two research sites located a few kilometers north of Cananea, Sonora, in the headwater portion of the Upper San Pedro Basin (Figure 1).

The Zapata Site (Figure 2) represents the Plains Grassland biotic community which still comprises much of the upland watershed in the Mexico part of the Basin. This is the largest expanse of native Plains Grassland in all of Sonora and is of considerable ecologic and economic value. The grassland retains much of its original character but in recent decades has experienced heavy grazing pressure by local ranchers.

The Riecito Site (Figure 2) is located in the center of the Basin a few kilometers west of the Zapata Site in an area dominated by dense mesquite woodland, or "mezquital." This bottomland site likely converted from grassland and riparian vegetation to mesquite woodland due to heavy overgrazing in the past century. Although lacking in biodiversity, these mesquite woodlands supply building materials and fuel for the local economy.

 

Figure 2. Location of SALSA Mexico experimental sites.

The objective of this investigation are:

1- to examine the effects of view angle on radiative surface measurements over semi-arid vegetation in the San Pedro basin.
2- To interpret nadir/off nadir temperature differences in terms of soil and vegetation characteristics.
3- To examine the possibility of inferring soil and vegetation temperatures using measurements of radiative surface temperature at two different view angles, i.e.., 0 and 45 deg using the directional radiative transfer model
4- To use these component temperature to estimate sensible heat using a two layer based approach.
5- Finally attempt to estimate sensible heat flux using ATSR data will be made.

SITES: Experimentation would be carried out at both the US site at Lewis Spring (Mesquite-grass association) and the mexican site at Cananea (Mesquite-bare soil association). Both sites are about 50 km from each other.

METHODS: Gradients in concentrations and isotopic compositions (18O) of CO2 and water vapour would be measured from a 10 m (mast/tower) at 5 different levels from the soil surface to 10m (i.e 4 to 5 m above the Mesquite canopy). Air would be sucked through a CO2-H2O infra-red gas analyser (LICOR 6262) to determine concentration gradients simultaneously through cold traps for determination of isotopic gradients. Sampling of soil and vegetation for further water extraction and isotopic analysis would also be performed at the two sites.
At both sites mast/tower would be equipped with additional instruments. At the mexican site: 3D sonic anemometer and Krypton hygrometer (Campbell system) for estimation of global fluxes using eddy correlation technique, IRT for surface temperature measurements and heat balance technique device for sap flow measurements. Similar instruments would be at the US site, but Bowen ratio technique would be used instead of Eddy correlation.
Isotopic analysis would be made using a mass spectrometer either at the university or Biosphere 2 facilities in Tucson.

COLLABORATION: These experiments have been designed and will be carried out by the following persons:

Dr. Ganghui Lin from Columbia University (Lamont Doherty Earth Observatory), at the Biosphere 2 research center.
Dr. D.G. Williams from University of Arizona (School of natural renewable resources), Tucson.
Dr. J.P.Brunel, from ORSTOM, Hermosillo, Mexico.
There will be also involvement of graduate and PhD students.
Participation from Dr. P.Mordelet from CESBIO, Toulouse, is expected sometime during the field campaign.

Large aperture scintillometry (LAS) measurements appear as a promising technique for estimating sensible surface heat fluxes over large and possibly composite areas. The method has already been validated over homogeneous surfaces by several teams (see for instance recent experiments described by McAneney et al. over fallow, or De Bruin et al. over wineyards...). Less work has been made in the case of composite areas, but recent experiments performed by INRA Bioclimatology laboratory in the South-East of France in the framework of the Alpilles/ReSeDA 1997 campaign (paper in preparation), as well as results obtained in Crete (Greece) by H. de Bruin at Institute of Meteorology, Agricultural Univ. Wageningen (oral communication at 'Up-Scaling' workshop, Wallingford 1996, UK) confirm the interest of LAS for directly retrieving fluxes over heterogeneous areas.

Nevertheless, the effect of the wind (direction in relation with the direction of the optical path, and direction variability) is not quite clear and to our knowledge has not been systematically investigated (McAneney et al., 1995 ; Hill et al., 1992).

10-14 /08 : installation of INRA instruments on the site
15-17/08 : intercomparison of instruments (3 days measurements along close parallel optical paths).
18-30/08 : measurements with 3 different orientations of the LAS (0, 45, and 90°).

The 3 LAS will be provided by ORSTOM (1) and INRA (2). The reference data will be provided by 2 3D sonic anemometers, and a meteorological station.

Remotely sensed data starting from 1972 show that grasslands in the Mexican portion of the San Pedro Basin have not only decreased in area but in diversity as well. Broad expanses of mesquite bosque have taken the place of the original grassland, with a resulting reduction in species diversity. Grassland vegetation is the backbone of the main economic activity in the area and most of the rural population heavily depends on it; hence, the importance of the analysis in terms of the current conditions of what to be done to conserve natural resources, and the practices needed to restore environmental problems.

Our hypothesis in the area is that extreme climatic events combined with poor management practices are responsible for vegetation changes. Overgrazing combined with a severe drought may have been responsible for the loss of the desirable vegetation (grasses).

Does hydrologic properties of soils play a major role in the vegetation changes?, what happens once vegetation is removed?

Two experimental sites were selected: one in Ejido José María Morelos, mesquite bosque ("mesquital") as a disturbed grassland, and the other in Ejido Emiliano Zapata, with a natural grassland. Both were instrumented with runoff and rainfall rate devices.

In both sites we will measure:

1. Rainfall rate, using a recording rainfall
2. Runoff rate, using a flume with a water level recorder
3. Soil Moisture
4. Vegetation attributes (density and cover)
5. Hydrologic properties of soils: saturated hydraulic conductivity, and water retention curve

First year: data collection to conduct a water balance and runoff model callibration (SWRRB-WQ, and possibly SWAT depending on resources and personnel).

Simulation of possible scenarios of reclamation and conservation using the hydrologic model to assess the possible techniques (mechanical treatments, and plant attributes effects on water retention and erosion reduction).

Second year: Treated watershed with conservation and reclamation practices. The grassland would requiere conservation measures (including mechanical practices and cover increase by the seeding of other native grasses or associated plants) and the mesquite bosque with reclamation measures (involving the removal and utilization of mezquite debris, mulching and planting of mixes of grasses, forbs and annual plants to induce grassland restoration).

Third year: Evaluation of performance of conservation and reclamation practices.