INSTRUMENTOS PARA LA PLANIFICACIÓN EN MATERIA DE

The following conclusions can be summarised from this chapter:

It has been demonstrated that the application of the atmospheric correction technique over land can improve the accuracy of soil moisture retrieval. If an atmospheric correction is not applied to the microwave emission, there may be considerable error in the soil moisture retrieval. Although in this study the error without atmospheric correction is -0.01 in soil moisture which is still within the 0.02 accuracy needed for climate application, however, such error increases as soil moisture increases (and emissivity decreases) and the retrieval of soil moisture within 0.02 accuracy becomes difficult, if not impossible. Therefore the benefits of applying the new atmospheric correction technique to soil moisture retrieval are more noticeable for wet and smooth surfaces when the microwave surface emissivities have low values and the atmospheric contributions are proportionally greater.

Although the accuracy needed for soil moisture in climate application is 0.02, it is feasible to do better than this. It has been shown that, in the absence of vegetation cover, it is possible to measure the land surface temperature to 1°K uncertainty error at night with AVHRR. Therefore, achieving the 0.005 accuracy of soil moisture or better is possible but will be limited to the accuracy of the soil model used.

Although it is difficult to draw firm conclusion from only four measurements of soil moisture, the result of soil moisture retrieval from TP are very encouraging. The comparison study between the differences from field soil moisture measurements and retrieved soil moisture from TP show a standard deviation of 0.008 which is close to the predicted error of 0.005 from the stochastic analysis. The approach described in this chapter will allow operational soil moisture retrieval over arid and semi-arid regions, provided that a nadir looking radiometer is used, minimising the effect of vegetation.

Although the application of the new atmospheric technique in this study used high frequencies channels (i.e. 18 GHz and 37 GHz) which are more affected by the atmosphere, the application of this technique should also be useful for low frequencies which are more desired for soil moisture applications. As simulations

*

of chapter 4 showed that even for 1.4 GHz, there is an error of 0.01 for wet and smooth surfaces when the surface emissivities are low. Although such error is still within the 0.02 soil moisture accuracy for climate studies, it is feasible to get a 0.005 accuracy of soil moisture when atmospheric correction is considered and surface temperature is measured with an accuracy ~ 1 K.

The field data was limited in scope with few night time measurement of soil moisture and only limited measurements of surface temperatures. More measurements of field data are required to test the method fully. However, a more complete field program to test the technique faces the difficulties of the long period between satellite passes and hence few data points simultaneous with satellite overpasses.

Although AVHRR can measure land surface temperature to within IK, the large temperature gradients, in desert regions mean that the IR and microwave measurements must be within 30 minutes. NOAA satellite lack a microwave radiometer so ATSR is the only instrument that can do this. Unfortunately the ATSR/M radiometer does not have an 18 GHz or lower frequency channel. However, Earth Observation System (EOS) planned in the late 1990s will provide both infrared and microwave radiometers (see table 1.1). The EOS microwave radiometers have low frequencies channels which are important for soil moisture retrieval (see table 1.2).

Chapter 7

Conclusions

7.1 Introduction

The nature of the climate system has been reviewed (chapter 1). The importance and unique role of remote sensing towards climate studies has been discussed. The surface microwave emissivity has been identified as a valuable parameter for the climate research, as it depends on several important geophysical parameters (notably soil moisture, Rowntree, 1993). The effect of the atmosphere on surface microwave emission as measured by satellite radiometer is addressed. The few previous studies investigating this problem has been reviewed.

A necessary understanding of the physics of microwave emission, including assumptions made and constraints applied, were described (chapter 2). The microwave surface emission models for ocean and soil used in the validation and application o f the atmospheric correction technique have been reviewed and examined. The effect of the atmosphere on the satellite measurement of microwave emission has been demonstrated through the radiative transfer process for clear sky conditions.

The characteristics of the satellite microwave and infrared radiometers used in this research have been reviewed. The implication of these characteristics for the technique described in this thesis were addressed in chapter 3.

A novel atmospheric correction technique to correct for atmospheric effect (clear sky conditions) and retrieve microwave surface em issivity using sim ultaneous

measurements from passive and infrared radiometers has been proposed in chapter 4 (A1 Jassar et al. 1995). A simulation study using an atmospheric radiative transfer model, together with generic atmospheres covering all seasons, have been used to investigate the atmospheric contribution to the microwave signal from space for a range of all possible terrestrial emissivities. It was found that the atmospheric effect, in particular, at lower surface emissivities contribute to a major source of errors. Predictions from an ocean surface emission model were compared with corrected microwave emissivities in order to validate the atmospheric correction technique (chapter 5). The application of the atmospheric correction technique over land to improve the accuracy of soil moisture retrieval has been demonstrated, and soil moisture measurements retrieved from satellite were compared with contemporaneous ground data (chapter 6).

In this chapter, the achievements of this research are summarised, and the contributions towards climate studies are addressed. Finally, recommendations for future work are made.