Use of Imaging Spectroscopy to Assess the Impact of Land Use Changes in a Semi-arid Karstic Landscape: Los Monegros, Spain

Significant land use changes, mainly in the form of changing agricultural practices, are transforming a semi-arid karstic environment in NE Spain that encloses numerous saline lakes (playa lakes), forming a unique habitat. The introduction of irrigation systems in an area characterized by a semiarid climate, closed basin hydrology, and karstic geology of evaporitic sediments, is changing the water balance and soil properties of the playa lakes and surrounding areas. As a result changes in the distribution and magnitude of salt affected soils have been observed through the analysis of a time series of satellite data. In this work, we offer a refinement of the methodology used to detect and monitor changes in soil composition and status by utilizing hyperspectral and multispectral data. Soil endmembers from a spectral library obtained in a similar semi-arid environment were applied to the study site by means of the Spectral Angle Mapper classifier. The spatial distributions of three soil groups were obtained and related to land use changes affecting the playa lakes and their environments. The soil groups showed different contents of soluble salts and their mineralogy is closely correlated to the underlying geological units. This exercise illustrates the transferability of spectral information across time, space and sensor type. Future work will include the creation of a site specific spectral library from two sets of hyperspectral HyMap images taken during the wet and dry season, so that short term seasonal changes can be differentiated from long term progressive changes in the soil and water interface of the playa lakes and surrounding areas. INTRODUCTION The Los Monegros area of NE Spain is a semi-arid region that is experiencing significant land use change due to the implementation of an extensive irrigation system that is converting semi-natural vegetated areas into arable land (i,ii). This region is characterized by its semi-arid climate, sparse vegetation and shallow as well as poorly developed soils. The study area lies within the central part of the Ebro basin and its landscape is typical of a karstic environment. Current agricultural policy encourages farmers to plow semi-natural areas irrespectively of their profitability. The effect of plowing, combined with the subsequent land use changes, may trigger or accelerate land degradation processes, such as water and/or wind erosion, soil alkalinization or salinization, and vegetation loss (i, ii). Numerous small playa lakes are found within a plateau area in the study site (Figure 1). These lakes form in small karstic depressions by the dissolution of evaporitic subsurface layers, mainly gypsum and limestone. These dolines or sinkholes, which are the result of collapsing surface sediments, are subsequently filled with water from shallow aquifers (iii). The lakes are saline due to the effect of surface water evaporation leading to the precipitation of salts at the land surface, and they form a very particular and fragile environment with a unique habitat adapted to this ecosystem. The playa lakes are usually dry in the summer and fill with water during the rainy season when the water table rises (i). A time series of Landsat 5 TM images of the Los Monegros area covering a time period of 13 years (1984, 1991 and 1997) was used by (ii) for detecting and mapping land use/cover changes and © EARSeL and Warsaw University, Warsaw 2005. Proceedings of 4th EARSeL Workshop on Imaging Spectroscopy. New quality in environmental studies. Zagajewski B., Sobczak M., Wrzesień M., (eds) their effect on soil salinity in this karstic environment. The author (ii) was able to demonstrate that the implementation of large-scale irrigation systems in such an environment has triggered an increase in areas affected by soil salinization problems (gypsum and calcite rich soils), which are detectable on the land surface by multispectral sensors. At the same time the author suggests that these land degradation processes may be controlled by the geology in this area (lithology and structures). Studies carried out in other dryland regions have shown a close relationship between the expansion of irrigated lands and increased rates of soil and water salinization (iv, v). Figure 1: ASTER image (29 May 2003) of the study area in Los Monegros (bands 2, 3, 1 as RGB). The image shows a plateau area with numerous playa lakes surrounded by irrigated (bright green) and non-irrigated agricultural fields and bounded in the south by the Ebro river escarpment. The image covers an area of 15 km x 12 km. The present work offers a refinement of the methodology implemented in an earlier work (ii), by utilizing a spectral library developed for a semi-arid wetland zone in La Mancha, Spain (vi). The spectral library is based on airborne hyperspectral data (DAIS 7915) combined with field spectral information (ASD FieldSpec Pro spectrometer) for validation, and was successfully applied to the problem of mapping salt affected soils and vegetation in a wetland area in La Mancha with similar climatic conditions to those in Los Monegros (vii). In the present work, the spectral library of the La Mancha site is used to obtain high spectral resolution information of salt affected soils in order to implement these spectral signatures to a lower resolution multispectral image (ASTER) of the Los Monegros site. The purpose is to improve the detection and classification of soils associated to playa lakes and/or sink holes so that their degradation stage can be monitored over time based on a time-series of multispectral images such as those acquired by the Terra ASTER or the Landsat TM/ETM+ sensor. A second objective of this paper is to outline a methodology for future work that will include the use of multi-temporal hyperspectral data from the airborne HyMap sensor. At this point we have completed the first stage of our workflow where multispectral data have been successfully used to discriminate and monitor salt affected soils in a semi-arid environment. We have also acquired preprocessed hyperspectral data within the framework of the HyEurope program in 2004 that was jointly organized by the German Aerospace Centre (DLR) and HYVISTA Corporation. This data acquisition is in preparation for the development of a methodology that will encompass multispectral as well as hyperspectral data for assessing the effects of changing agricultural