Hydrological processes on large flatlands: case study in the northwest region of Buenos Aires Province (Argentina)

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1 Remote Sensing and Hydrology 2000 (Proceedings of a symposium held at Santa Fe, New Mexico, USA, April 2000). IAHS Publ. no. 267, Hydrological processes on large flatlands: case study in the northwest region of Buenos Aires Province (Argentina) EDUARDO KRUSE Universidad Nacional de La Plata and CONICET, Facultad de Ciencias Astronômicas y Geoftsicas, Paseo del Bosque, La Plata 1900, Argentina kruse(a).fcaglp.edu.ar JUAN ALBERTO FORTE LAY CONICET, Avenue Paseo Colon 751, Buenos Aires, Argentina JOSE LUIS AIELLO CONAE and Universidad Nacional de La Plata, Avenue Paseo Colon 751, Buenos Aires, Argentina ADRIAN A BASUALDO & GERMAN HEINZENKNECHT FECEACOP, Avenue Corrientes 119, Buenos Aires, Argentina Abstract. The main features of the northwest region of Buenos Aires Province (Argentina) are its small topographic slope, the lack of a drainage network and the presence of permeable sediments near the surface. As a consequence, there is no fluvial network and the lagoons or ponds located in the depressions are important. Landsat 5 TM images have shown good contrast between water and land areas. All available scenes between August 1998 and August 1999 were used in a multitemporal analysis. The areas of the lagoons, obtained by satellite images, in each period were compared with hydrological variables measured in situ. The results show a relationship between variations of the water balance (precipitation - évapotranspiration) and fluctuations of the water table and lagoon areas. Key words agricultural system; Buenos Aires Province (Argentina); flatlands; hydrological processes; image classification; image geo-referencing;lagoon areas; Landsat; water table fluctuations INTRODUCTION Buenos Aires Province in Argentina is a great plain with an area of approximately km 2. It contains several sub-regions with different hydrological features, one of them being the northwest region. The social and economic activities in this region are strongly influenced by the availability of water resources. The study of their variability is an important issue for the analysis and prediction of the behaviour of this productive system. A characteristic of this flatland is the low morphological energy of its terrain. As a consequence, the natural storage (depression and groundwater storage) and vertical movement of water (évapotranspiration and infiltration) dominate over horizontal movement (surface and subsurface runoff). Therefore, it is essential to measure the water balance and the vertical exchange of water (UNESCO, 1983). To do this,

2 532 Eduardo Kruse et al. variations in water table and depression storage should be measured in addition to conventional observations. In this context, the satellite image is an important tool for measuring the fluctuations of depression storage. This work shows the importance of the monitoring of hydrological stages by multitemporal image analysis along with hydrological variables in the region. REGIONAL FEATURES The area is a great plain with minimal slope (-0.1 m km" 1 ). Minor topographic forms that are important hydrologically can be differentiated within the general morphology. There are slight linear elevations alternating with depressions and "canadas" that give a relief of soft, somewhat aligned crinkles, the typical forms of a palaeodune environment. The region shows elongated ridges with a prevailing north-south direction throughout, and dimensions of about 10 km by 2 km. From a hydrogeological perspective, the region is covered by recent permeable sediments. Fine sands and silts form the dunes and several overlying aeolian cycles have been recognized (Aiello et al, 1995). In the interdune areas, finer sediment with lower relative permeability has been found. It is an arheic environment but floods occur in the low lands during humid periods (Kruse et al, 1993). At the surface, sandy aeolian sediments of thickness ranging from a few centimetres to four metres were found. They lie over silt with varying proportions of sand and clay, and usually calcareous materials, known as "sedimentos pampeanos" (Fidalgo et al, 1975). The mean annual rainfall in the region is about 850 mm year" 1 and the mean temperature 16 C. Rainfall patterns in the region show alternating dry and humid periods with the first half of the century drier than the second (Fig. 1). YEAR Fig. 1 Ten year mean annual rainfall (solid line) and polynomial trend (dashed line).

3 Hydrological processes on large flatlands: northwest Buenos Aires Province (Argentina) 533 METHODOLOGY Using traditional methods of satellite image analysis, the variations of lagoon areas were measured. These variations were obtained from cloud free LANDSAT 5 Thematic Mapper (TM) images between August 1998 and August 1999 (9 September 1998, 12 November 1998, 31 January 1999, 21 April 1999, 30 May 1999, 26 July 1999 and 27 August 1999). The images were provided by CONAE (Comision Nacional de Actividades Espaciales), under an agreement between this institution and FECEACOP (Federaciôn de Centras de Acopiadores de Argentina). The procedure used for estimation of the lagoon areas included the following stages: (a) Image geo-referencing, where each image was corrected by conventional methods using ground control points from a topographic map. These ground point controls were distributed evenly around the image and the final error was 30 m. This stage includes transformation of the image coordinates given arbitrarily by the software used (LDRJSI), to the Gauss Kruger projection system, which is used in Argentina. This procedure allowed the overlapping of the images with cartographic material and the use of the images in a geographical information system. (b) Image classification was accomplished through the use of a histogram threshold over TM band 7 data. This procedure allowed the generation of a new image where each pixel was classified according to its digital number (grey level). The threshold selection was supported by field work. TM band 7 was selected because of the important water absorption peak in this region. The TM5/TM2 ratio was also used to discriminate the water-land boundaries. This ratio was also used with good results in nearby areas (Mehl, 1991). In the histogram analysis a significant peak was observed for the digital number between 0 and 10 in all the analysed cases and this grey level interval was associated with lagoon areas. As a consequence, we proceeded to generate the classification using the value 10 as the selected threshold. The resulting images have two distinct classes, one for lagoon areas and the other for land areas. According to the histograms obtained from the different images, the threshold 10 is adequate for all the images since land areas always have a grey level that is higher than 50. The final results were verified by means of field observations. This procedure allows the definition of the increase and decrease of the lagoons' areas. The areas, obtained from satellite images, in each period were compared with hydrological variables measured on the ground. RESULTS The regional influence of the relationship between precipitation and water table variations has been recognized (Kruse et ah, 1993). This allows us to establish different hydrological responses, according to the analysis in humid and dry periods. August 1998-January 1999 was a relatively dry period. As a consequence of the low rainfall, the water table deepened and some existing depressions became dry. Figure 2 shows the relationships of lagoon areas derived from imagery and water table fluctuations.

4 534 Eduardo Kruse et al. Sep- Oct- Nov- Dec- Jan- Feb- Mar- Apr- May- Jun- Jul- Aug WWm lagoon area water table 1 * water table 2» water table 3 Fig. 2 Lagoon area variations and water table fluctuations. Dates: satellite images. 12 November January April 1999 Fig. 3 Lagoon area variations between 12 November 1998, 30 January 1999 and 21 April 1999 (white = water). Variations between the lagoon areas detected in the November 1998 and January 1999 images can be observed in Fig. 3. The water table depth, as much in its magnitude as in its intensity, shows a relationship with the reduction of the lagoon areas. In March 1999 an extreme rainfall event occurred, where water input to the system exceeded the output. A rise in the water table and an increase in lagoon area were noticed. Thus, the relationship between water excess in the hydrological balance, soil water content, water table fluctuations and lagoon areas are good indicators of the hydrological state. On the other hand, knowledge of the current hydrological state and the historical data of the other indicators allow us to predict future hydrological scenarios.

5 Hydrological processes on large flatiands: northwest Buenos Aires Province (Argentina) 535 This region has a productive agricultural system. Significant crops include wheat, corn, sunflower and soybean. For agriculture there are two factors to consider concerning the influence of the hydrological processes. One of them is linked to operational problems and the other is related to the water requirements of the crops. In humid periods the magnitude of the flooded areas associated with the shallow depth of the water table directly affects sowing and harvesting operations. It determines the area available for cultivation. In a dry period, the depressions are empty as the water table increases its depth. Under these conditions the soil water content is low and may reach drought levels. The analysis of the sowing and harvesting times in relation to water demand allowed us to establish critical dates when the hydrology becomes significant for agricultural activities. During such periods the analysis of satellite images should be performed. CONCLUSIONS The alternating dry and wet periods in the region have a noticeable impacts on the hydrology and consequently agriculture. During a dry period, the deepening water table makes satisfaction of agricultural demands for water difficult. During humid periods the water table is shallower, usually producing flooding and negatively affecting agricultural production. The water table is often very close to the surface, which indicates that surface waters (lagoons) and groundwater are strongly related, so they should be treated as a single unit. Landsat 5 TM data can be used to track lagoon area fluctuations. These fluctuations are important indicators of hydrological behaviour. Acknowledgements This work was supported by the Federaciôn de Centras de Acopiadores (FECEACOP), Argentina, and the satellite images were provided by Comision Nacional de Actividades Espaciales (CONAE). REFERENCES Aiello, J. L., Kruse, E., Kuba, J. & Forte Lay, J. A. (1995) Diagnôstico, moniloreo y pronoslico de las anomalias hidricas en el noroeste de la Provincia de Buenos Aires. Academia Nacional de Geografia, Buenos Aires, Argentina. Fidalgo, F., De Francesco, O. & Pascual, R. (1975) Geologia superficial de la Llanura Bonaerense. In: VI Geologico Argentino. Relatorio, Asociacion Geolôgica Argentina, Buenos Aires, Argentina. Congreso Kruse, E., Aiello, J. L. & Forte Lay, J. A. (1993) Aspectos hidrologicos en el Oeste de la Provincia de Buenos Aires y Este de La Pampa, Repûblica Argentina. Anais I Simposio de Recursos Hidricos del Cono Sur. Gramado (Brasil) 2, Mehl, H. (1991) Evaluaciôn de procesos dinâmicos sobre la base de datos operacionales de sensores remotos. Disertaciôn Inaugural, Facultad de Geociencias, Universidad Ludwig Maximilian, Munich, Germany. UNESCO (1983) Hidrologia de las Grandes Llanuras (Actas del Coloquio de Olavarria), vols 1, 2 & 3. Buenos Aires, Argentina.