Despite being an experimental mission, as far as I know, SMOS is providing good results in such zones... Techniques of downscaling can be used to have high resolution maps...

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5756664&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F36%2F5997355%2F05756664.pdf%3Farnumber%3D5756664

for soil moisture analysis ESA's recently developed soil moisture data with a 10km cell size could be an adequate solution. It provides daily blended data from 1978 although, unfortunatly, only goes (at the moment) until 2010

the problem is that I have a small area (150 km2) with soil moisture monitoring data

As allways, its dependent on the objective of your research. if you look for a stable temporal resolution with minimal or no costs the spatial resolution of the data will allways be a major issue. As sugested before by other colleagues, Radarsat could be an adequate solution (when looking for high spatial resolution). However, it presents large costs of aquisition depending of the targeted time frame and its status (archived or not). Additionaly, although you might achieve a better spatial resolution, a loss of temporal resolution is expected.

In alternative, if you are just looking for a comparative notion of moisture availability on the surface, radiometer data (e.g. MODIS or Landsat) could be a reliable alternative.

The ESA's SMOS mission, launched in November 2009, is devoted to soil moisture monitoring. It provides soil moisture data at 40 km spatial resolution with an accuracy of 0.04 m3/m3. If the resolution is too coarse for your application, there is the possibility of downscaling SMOS soil moisture data to 1 km --in line with Justino's comment above. In this link you can browse and access soil moisture maps at 1 km spatial resolution covering the Iberian Peninsula, as well as other products:

http://cp34-bec.cmima.csic.es/

Soil moisture maps at 1 km spatial resolution over any area of interest could be obtained using the same downscaling approach. CP34-BEC facilities are ready to do so upon demmand. Data is available from 2010 to present.

Not yet flying, but keep an eye out on the SMAP (Soil Moisture Active Passive) mission, currently scheduled for launch next Fall. A combination and passive and active remote sensing methods is expected to lead to improved soil moisture retrievals. See: http://smap.jpl.nasa.gov/

Nowadays, satellite soil moisture products from ASCAT, AMSR-E and MIRAS (SMOS) sensors are freely available. Data have spatial resolution of 25 km and daily coverage.

ASCAT: http://www.eumetsat.int/Home/Main/News/Features/708786

AMSRE: http://nsidc.org/data/docs/daac/ae_land3_l3_soil_moisture.gd.html

MIRAS (SMOS): http://www.esa.int/esaMI/smos/

The temporal stability properties of soil moisture observations allows to use data with a coarse resolution for estimating soil moisture over smaller areas, and viceversa.

Try to get and use this data. Likely, you will obtain unexpected good results, even though you are making comparisons at different spatial scales.

Indeed, some good correspondence between satellite retrievals and in-situ observations can be obtained. See:

Albergel et al. 2009: An evaluation of ASCAT surface soil moisture products with in-situ observations in Southwestern France. Hydrol. Earth Syst. Sci., 13, 115–124.

(http://www.bartalis.com/publications/albergel_etal_2009.pdf)

Prigent, C., F. Aires, W. B. Rossow, and A. Robock (2005), Sensitivity of satellite microwave and infrared observations to soil moisture at a global scale: Relationship of satellite observations to in situ soil moisture measurements, J. Geophys. Res., 110, D07110, doi:10.1029/2004JD005087.

Actually, many studies were already published on the comparison of satellite and in situ soil moisture observations.

I can suggest you some of mine (:-), available on RG), but in the references you can find a long list:

Brocca, L., Hasenauer, S., Lacava, T., Melone, F., Moramarco, T., Wagner, W., Dorigo, W., Matgen, P., Martínez-Fernández, J., Llorens, P., Latron, J., Martin, C., Bittelli, M. (2011). Soil moisture estimation through ASCAT and AMSR-E sensors: an intercomparison and validation study across Europe. Remote Sensing of Environment, 115, 3390-3408, doi:10.1016/j.rse.2011.08.003.

Lacava, T., Matgen, P., Brocca, L., Bittelli, M., Moramarco, T. (2012). A first assessment of the SMOS soil moisture product with in-situ and modelled data in Italy and Luxembourg. IEEE Transaction on Geoscience and Remote Sensing, 50(5), 1612-1622, doi:10.1109/TGRS.2012.2186819.

Lacava, T., Brocca, L., Faruolo, M., Matgen, P., Moramarco, T., Pergola, N., Tramutoli, V. (2012). A multi-sensor (SMOS, AMSR-E and ASCAT) satellite-based soil moisture products inter-comparison. Proceedings 2012 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), 1135-1138, doi:10.1109/IGARSS.2012.6351348.

Brocca, L., Melone, F., Moramarco, T., Wagner, W., Hasenauer, S. (2010). ASCAT Soil Wetness Index validation through in-situ and modeled soil moisture data in central Italy. Remote Sensing of Environment, 114 (11), 2745-2755, doi:10.1016/j.rse.2010.06.009.

think that ENVISAT good satellite , you will show in good condition ,small perturbation of deformation incha Allah .