SOIL WATER ASSESSMENT USING A P-BAND SCATTEROMETER AND ERS-2 SAR

D. G. Blumberg (1), J. Ben-Asher(1) ,G. Ronen(1), M. Sprintsin(1), V. Freilikher(2), L. Volfson(2), and A. Kotlyar(2).

(1) Ben-Gurion University of the , P.O. Box 653, Beer Sheva 84105, . E-mail: [email protected] (2) Bar-Ilan University , , Ramat Gan, Israel

ABSTRACT/RESUME

Soil water content is an important component that influences meso- and micro scale processes. The agricultural capacity of a site is directly affected by soil water content and is especially important in arid regions. Furthermore, climate studies face the never ending problem of trying to lay a hand on climate surface conditions. Many of these are not measured continuously and thus are not available. One of the more important parameters is soil water-content. In this study we mapped soil water content in the Negev Desert using ERS-2 with multiple look directions and utilized a P-band scatterometer. Results show water content values ranging from 4-20% volumetric, with a high correlation to ground measurements.

1. INTRODUCTION

Soil water content is an important component that influences meso- and micro scale processes. The most developed models for extracting soil water-content are those that utilize polarimetric SAR data. However, the wavelengths most insensitive to surface roughness and thus, most sensitive to soil water content are the longer wavelengths (i.e., P-band). These are not readily available in Space and therefore different methods need to be used at this time. The use of archival spaceborne data provides the possibility of reprocessing data acquired in the past and reconstructing missing surface data. However, even when such systems become available they will lack the historic perspective and will not be suitable for reconstructing climatic surface conditions from the recent past. The use of microwave energy for retrieval and mapping of soil water content is based on microwave response to the complex dielectric constant of soils. The Fresnel reflectivity determines that the reflectivity for light polarized in (parallel to) the plane of incidence is:

2 é1- e r ù R0 = ê ú [1] ëê1+ e r ûú

in which er is the dielectric constant of the soil-water mixture and the dielectric constant of air is assumed to be1. The reflectivity of light is dependent upon the angle of incidence and the polarization of the electromagnetic wave. The Fresnel reflection coefficient in this case is:

2 cosq - e -sin 2 q [2] R = r H 2 cosq + er -sin q

2 2 e r cosq - e r -sin q R = [3] V 2 e r cosq + er -sin q

where RH and RV are the relative reflection coefficients for horizontally and vertically polarized waves respectively.

2. METHODS AND MATERIALS

1. 1 Sensors Two sensors operated remotely were used for this project. The first is a P-band continuous wave frequency modulated (CWFM) scatterometer (l=68cm). This scatterometer operates at nadir with a wide antenna diagram of 10°.and operating at nadir; the second is the European ERS-2 SAR operating at C-band (l =5.3 cm and VV polarization). For the ERS-2 we had at hand some 20 images spanning a period from 1996 to date. The scatterometer was mounted several times on a cherry picker for experimentation and twice on a Cesna 174 aircraft for mapping campaigns.

1. 2 Study area

The study area was located in the Negev Desert in Southern Israel. Three major study areas were selected for this study. The sites include the experimental farms at Yotvata , , and Wadi Mashash. The first two were used to study the specific response of the scatterometer to changes in the soil water content and the latter to map the spatial variations in the water content as a function of season and climatic conditions using the ERS-2 imagery. 1. 3 field measurements At Ashalim and Yotvata soil water content was measured using a Tktronix TDR and by taking gravimetric soil samples. All these were done for the near surface layer of 0-5cm. For the flight experiments and the ERS-2 overflys concurrent field measurements included measuring soil water content at 4 depths, every 15 cm, at four locations along the scatterometer and ERS-2 footprints. The soil samples were taken using airtight “Gerber baby food” jars. The soil samples were dried at 105° for 24 hours and the gravimetric and volumetric water contents were determined.

3. RESULTS

The first set of experiments was aimed at studying the wetting and drying cycles of the sandy soils at Yotvata and Ashalim. The soil was irrigated in a controlled manner and the scatterometer was mounted on a cherry picker 8 m above the surface. All the scatterometer and water content data were normalized to bring them onto a single scale. The representation of the soil water content is through the saturation percent:

q - q q sat = i res [4] q sat - q res

where qi is the soil water content measurement and q sat,q res refer to the saturation water content and the residual water content respectively. The scatterometer data were normalized similarly by:

R - R Rrel = i min [5] R max - R min where Ri is the relative reflection and max and min subscripts refer to the reflection from the soil at maximal water content when saturation is achieved and minimal water content (or residual water content) respectively.

Figure 1. shows the scatterometer response (black dots) compared to the change in soil water content presented as a normalized saturation percent (bars) as a function of time.

Gravimetric Water Content & Scatterometer Aplitude vs Time

1.0

.8

.6

.4 Normalized Values

.2

0.0 0 20 40 60 Time (minute)

Gravimetric Water content Time vs Scatterometer Amplitude

Figure 1. The response of the scatterometer (in black dots) and the changes in saturation water percent as a function of time during the wetting cycle of a sandy soil.

Results from the Scatterometer flights were mapped using contouring software. The fuselage was used as a reflector and there was a fear of cross talk between the two antennas. To determine cross talk effect, we first flew the aircraft over a large water reservoir (used as a fish pool) near for validation.

Results show 100% water content for the water reservoir with small depressions in between swaths related to the interpolation procedure. Interestingly, an increase in soil water-content is observed in the scatterometer map nearby a reclamation pond. This is caused by blockage of water flowing in the subsurface by the pond walls .

The ERS-2 data were applied by using several models for extracting soil water content. Only one is mentioned here. The method we used here is that of Blumberg and Freilikher ([1-3]) which utilizes two passes occurring within approximately 12 hours. The two passes result in multiple look directions providing the capability for discerning the dielectric and the roughness affects on the radar brightness. The test site was the Mashash water shed which is also a test farm for Ben-Gurion University. Overall we find that the soil water content in the summer months is ca 4% and in the winter months can reach nearly 20%. These results are consistent with measurements of soil water content in similar soils (Loessy soils) and similar climate as measured at the Ben-

Gurion University campus and similar to sporadic sparse measurements taken at the Mashash site. Spatial variability of the soil water content is much larger in the summer months than in the winter months and the highest water content is found in the dry wash area which drains the Mashash water shed. 4. SUMMARY

A variety of microwave methods are being used to map soil water content in the Negev desert. Presented here

are some of the methods that include P-band scatterometery and ERS-2 C-band SAR. All results are very

consistent with field measurements and the correlations are high. While these methods have yielded very good

results thus far a lack of polarimetric data and longer wavelength spaceborne data (such as P-band) for

discerning roughness and dielectric effects.

5. REFERENCES

1. Blumberg, D.G. and V. Freilikher, Soil water-content and surface roughness retrieval using ERS-2 SAR data in the Negev Desert, Israel. Journal of Arid Environments, 2001. 49(3): p. 449-464. 2. Blumberg, D.G., et al., Subsurface microwave remote sensing of soil-water content: field studies in the Negev Desert and optical modelling. International Journal of Remote Sensing, 2002. 23(19): p. 4039-4054. 3. Blumberg, D.G., et al., Soil moisture (water-content) assessment by an airborne scatterometer: The Chernobyl disaster area and the Negev desert. Remote Sensing of Environment, 2000. 71(3): p. 309-319.