Doñana National Park (DNP) extents over 543 km2
of the colmated Guadalquivir River paleo-estuary, in southwestern Spain. The area, considered the largest European habitat for migrating waterfowl, was declared a National Park in 1969, a Biosphere Reserve under the UNESCO Man and the Biosphere Programme in 1980, a Wetland of International Importance under the Ramsar Convention in 1982, a Special Protection Area under the European Union Directive on the Conservation of Wild Birds in 1988, and a UNESCO Natural World Heritage Site in 1994 [1
]. About half of DNP’s extension is marshland area, which experiences annual cycles of inundation and depletion. While flooded, Doñana marshes are a vast area of shallow lakes of variable size and degree of spatial connectivity, mainly depending on the regional pluvial regime and on the time of year. The natural hydrological cycle of the marshes, which constitutes the strategic basis for the entire ecosystem functioning, has been threatened by a number of past and present human actions [2
]. The alteration of tributary water courses (channels, deforestation, pollution) and the intensive agricultural occupation of the marshes during the 20th century have deeply altered their hydrological scheme and reduced their extension from about 1,800 to 300 km2
]. At present, the water exchange between the marshes and the Guadalquivir River is managed by DNP’s authorities through a number of sluices located in an artificial 12 km long levee.
In order to restore the natural hydrological behavior of the marshes, Spanish authorities promoted in 1998 the project called Doñana 2005 [6
]. The Institut Flumen, at the Universitat Politècnica de Catalunya and the International Center for Numerical Methods in Engineering, developed within the framework of this project a numerical model of DNP marshes in order to predict the hydrodynamic response of the wetland to the proposed restoration actions [7
]. The rigorous calibration of the hydrodynamic model was assisted by field data from a network of in situ
hydrometeorological gauging stations and by synoptic observations of the marshes through remote sensing images. Thus, Doñana images of the Advanced Synthetic Aperture Radar (ASAR) sensor on board on the Envisat satellite of the European Space Agency (ESA) [8
], acquired since 2006 at different incidence angles and polarization configurations, enabled the calibration of different capabilities of the model, as well as the monitoring of the marshes’ flood extent evolution and the seasonal development of helophyte vegetation [9
Given the flatness of Doñana’s topography, sustained winds can significantly modify the hydrodynamics and water extent of the marshes. Wind stress, as the vertical transfer of horizontal momentum between the atmosphere and surface, is considered a major driving force for shallow water circulation and mixing [10
]. When sustained over a long period of time, it produces a setup of the water surface in the downwind direction. Various studies on shallow water bodies have documented the relationship between water level and local wind intensity, with short time lags of a few hours between wind speed changes and water level changes [12
]. In the presence of low terrain slopes, as occurs in most wetlands and estuarine areas, this wind setup can eventually modify the inundation patterns [15
]. If stratification occurs, persistent winds have also been identified as the cause of massive fish kills by upwellings of hypolimnetic water [16
Current hydrodynamic modeling of wetlands and shallow reservoirs is normally taken into account using 2D depth averaged models based on either wastewater treatment or overseas experience [17
]. Wind stress modeling over the wetland surface ranges from simple formulations using a constant wind drag coefficient [17
] to more theoretically sounded complex formulations [20
]. Despite wind being usually presumed as the major driver of water movement and studied phenomena such as seiche formation [17
] or water turbidity [21
], little discussion is found in the freshwater modeling literature about the choice of the wind stress formulation. However, wind drag modeling is still an ongoing discussion in coastal and marine engineering [22
This paper describes the implementation of the wind stress action into the hydrodynamic model of Doñana marshes with the aid of Envisat/ASAR imagery. Two ASAR observation opportunities of an isolated water body are used to calibrate the observed wind-induced water displacement by choosing the most suitable wind stress formulation in the literature. The chosen formulation is also verified for another wind event observed in a different location of the marshes.
The paper is organized as follows. Second section describes the study area, Doñana marshes, and specifically the modeled ponds. Section 3 describes the study data, composed of radar imagery and field hydrometeorological data. Section 4 contains the methodology of flood mapping and hydrodynamic modeling. Section 5 presents and discusses the modeling results of the wind stress formulations tested at two observed events of wind-induced water displacement. Finally, Section 6 contains main conclusions and recommendations.
2. Study Area
Doñana marshes are composed of a variety of temporary shallow (<1.5 m depth) water bodies extended over an area of 298 km2
on the right bank of the Guadalquivir River mouth (Figure 1
). They are located at the former estuary of the river, which was filled by fine fluvial clayey sediment resulting in an extremely flat topography, with a maximum elevation difference of 2.50 m in its entire area. The main water inputs in the marshes come from direct precipitation and small and irregular northwestern tributaries, since they are disconnected from the Guadalquivir River and its tributaries by artificial structures. The climate is sub-humid Mediterranean with Atlantic influence [23
], with mild winters (average temperature of 10 °C in January) and hot dry summers (24 °C in August). Mean annual precipitation is 537 mm [24
], and average flooding periods occur from early autumn to late summer, when the marshes completely dry up [25
]. On average, about 50% of the mean annual precipitation corresponds to the period from November to January, while less than 5% to the period from June to September [24
]. However, the intra and inter-annual variability of the precipitation are high, resulting in flooding periods of variable magnitude and duration. Inside the marshes, small altitudinal gradients play an important role in defining the topographical elements (Figure 1
), the water spatial and temporal distribution and the predominant vegetation communities.
In this work, two of the greater ponds (locally named “lucios”) of the marshes are selected for the calibration and verification of the wind stress effect on the water extent, both at the southern part of the marshes (Figure 1
). First, a wind-induced water displacement event observed at Membrillo pond is selected for calibration purposes (named “Membrillo event”). The pond’s size is about 11.5 km2
and has the main longitudinal axis oriented to the N-S direction. Using the chosen wind stress formula, one more event is simulated at the Ánsares pond (named “Ánsares event”), which has an area of about 7.5 km2
and the main longitudinal axis at the WSW-ENE direction.
Most of the Membrillo and Ánsares ponds are formed by clayey bare soil, and virtually no vegetation develops in them at any time of the year [27
]. Helophyte communities have colonized the pond’s slightly higher terrain, located at the western and northern ends of Ánsares and Membrillo, respectively. These communities are dominated by Scirpus maritimus
(Castañuela) and Scirpus litorales
(Bayunco). The Castañuela is an herb of height ranging between 0.6 and 1.0 m. The Bayunco is reed-like, and often taller than 1.0 m. Both species start emerging from the water surface towards the end of February. They experience rapid growth throughout the spring season and then dry out during the summer. Their brown stems stick out of the water surface when the marshes flood in autumn, but they progressively decay and sink by the spring.
This paper describes the implementation of the wind stress action into the two-dimensional hydrodynamic model of Doñana marshes with the aid of Envisat/ASAR imagery. Given the flatness of the Doñana’s terrain, remote sensing images provided valuable spatial data on the wind-induced water bodies’ displacement. This information could barely be achieved by point field measurements.
Five wind stress formulations in the literature were introduced in the marshes’ hydrodynamic model. On-site wind records were then used to simulate the water wind drag effect in an isolated water body, the Membrillo pond, by using the five formulations. Two ASAR observations of the Membrillo pond flooded area during the same wind episode were used to assess the modeling results.
The Van Dorn’s expression [52
] and the power law wind profile for neutral stability conditions were selected as the most appropriate wind stress modeling technique. This formulation requires less field data and computational effort than the Charnock-based expressions, and yielded satisfactory results with an overall F coefficient value of 0.77 [63
]. The chosen formulation was verified by simulating a different wind event at the Ánsares pond, for which on-site wind records and ASAR imagery were also available.
Wind-induced water displacements up to 2 km were satisfactorily reproduced by the selected formulation, and water level variations were modeled with an average absolute error under 11 mm at 10 min calculation time steps. The maximum water displacements took place in time intervals of 12 to 18 h, in response to moderate and rather frequent wind velocities, below 15 m/s. In the presented study cases, the inclusion of atmospheric stability considerations and the increase of the wind drag coefficient at low wind speeds led to no significant improvement of the modeling results, with an increment of the F value under 1%.
Future lines of research will aim at mapping vegetation biomass by using polarimetric SAR imagery. These vegetation data will be implemented in the hydrodynamic model to spatially reproduce bedload resistance to flow, surface wind shear-stress reduction and evapotranspiration water losses. Further efforts will focus on modeling and calibrating water quality parameters of ecological relevance, such as wind-induced sediment resuspension and turbidity.