Addressing the multi-scale lapsus of landscape : multi-scale landscape process modelling to support sustainable land use : a case study for the Lower Guadalhorce valley South Spain

"Addressing the Multi-scale Lapsus of Landscape" with the sub-title "Multi-scale landscape process modelling to support sustainable land use: A case study for the Lower Guadalhorce valley South Spain" focuses on the role of landscape as the main driving factor behind many geo-environmental processes at different temporal and spatial levels. LAPSUS is the name of the geomorphological model developed in this study and at the same time it is taken, with a certain degree of freedom, as a reference to the underestimated importance of landscape as cause and result of geomorphological processes.

The main objective of this research is to investigate the role of the landscape at different spatial and temporal levels (extension and resolution) in geomorphological processes (e.g. soil redistribution: erosion and sedimentation), focussing on the sustainability of land use within a representative Mediterranean landscape. Landscape is defined in terms of genesis, geomorphology, lithology/ soil, land cover, land use, and even land management (human factor).

The research area chosen for this study is located in the south of Spain, surrounding the village of Álora, in the central Guadalhorce river basin in the province of Málaga, Andalucía (Chapter 1). The area has a mean annual temperature of 17.5 °C and receives a mean yearly rainfall of 534 [mm], distributed mainly from October to April. This research area was selected as representative for a wide variety of Mediterranean environmental conditions in terms of a complex geological history resulting in a spatial diversification over short distances of morphology, lithology and active landscape processes ranging from tectonics, land use changes to land degradation.

The study is directed, from the beginning to the end, at different spatial and temporal extensions-resolutions, studying different landscape processes within their specific spatial and temporal boundaries (Chapter 1). The first step in this investigation is the understanding of the evolution of the landscape and the geological background of the research area (spatial extension 10 ²[km ²], temporal extension 10 ⁷[a], temporal resolution 10 ⁴to 10 ⁵[a], Chapter 2). The second step is the development of a multi-scale landscape process model LAPSUS, valid at different spatial and temporal resolutions (spatial extension 10 ³to 10 ⁵[m ²], spatial resolution from 1 to 81 [m], Chapter 3). The third step comprises the actual measurement of net soil redistribution rates at the landscape level using the ¹³⁷Cs technique. First, the applicability of this technique under the current Mediterranean conditions of the research area is evaluated (spatial extension 10 ³to 10 ⁵[m ²] Chapter 4). Secondly, net ¹³⁷Cs derived soil redistribution rates on the temporal resolution of years and decades is simulated and the monitored erosion and sedimentation patterns are compared with the possibilities of the LAPSUS model (spatial resolution 7.5 [m], Chapter 5). The fourth step is the evaluation of the soil-landscape context at the multi-catchment or basin scale with special attention to the effects of soil redistribution upon water availability for vegetation (spatial extension 10 ²[km ²], Chapter 6). The fifth step is the integration of landscape process modelling and changes in land use to evaluate on-site and off-site effects (spatial extension 10 [km ²], spatial resolution 25 [m], temporal extension 10 [a], temporal resolution 1 [a], Chapter 7). As a final step a synthesis of results, comments and evaluation of the research is done (Chapter 8).

Landscape evolution from a geological perspective.

Landscape evolution is the result of a variety of geomorphological processes and their controls in time. Tectonics, climate and sea level fluctuations have mainly controlled landscape evolution in the research area. Data is obtained and analysed from the Upper Miocene to present (Chapter 2). Consequently, geomorphological reconstructions are made using sedimentary evidence such as marine and fluvial deposits, as well as erosional evidence such as terrain form and longitudinal profile analysis. These reconstructions add information and constraints to the uplift history and landscape development of the research area. Main sedimentation phases are the Late Tortonian, Early Pliocene and Pleistocene. Important erosional hiatus are found for the Middle Miocene, Messinian and Late Pliocene to Early Pleistocene. Concerning the palaeo-landscape, this resulted in a relative large and elongated Tortonian marine valley filled up with complex sedimentary structures. Next a prolonged stage of erosion of these deposits and incision of the major valley system took place during the Messinian. In the Pliocene a short palaeo-Guadalhorce, in a narrow and much smaller valley existed, partly filled with marine sediments combined with prograding fan delta complexes. During the Pleistocene a wider and larger incising river system resulted in rearrangements of the drainage network. Evaluating the uplift history of the area, the tectonic activity was relatively higher during the Tortonian-Messinian and Late Pleistocene, while it was lower during the Pliocene. Relative uplift rates for the study area range between 160-276 [m Ma ^-1] in the Messinian, 10-15 [m Ma ^-1] in the Pliocene to 40-100 [m Ma ^-1] during the Pleistocene.

Multi-scale landscape process modelling.

Once the geological background is understood, the development and testing of a landscape process model is undertaken (Chapter 3). Since resolution effects remain a factor of uncertainty in many hydrological and geomorphological modelling approaches, an experimental multi-scale study of landscape process modelling is presented, with emphasis on quantifying the effect of changing the spatial resolution upon modelling the processes of erosion and sedimentation. A simple single process model is constructed and equal boundary conditions are created. The use of artificial DEMs eliminates the possible effects of landscape representation. Consequently, the only variable factors are DEM resolution and the method of flow routing, both steepest descent and multiple flow directions. An important dependency of modelled erosion and sedimentation rates on these main variables is found. The general trend is an increase of erosion predictions with coarser resolutions. An artificial mathematical overestimation of erosion and a realistic natural modelling effect of underestimating re-sedimentation cause this. Increasing the spatial extent eliminates the artificial effect while at the same time the realistic effect is enhanced. Both effects can be quantified and are expected to increase within natural landscapes. The modelling of landscape processes will benefit from integrating these types of results at different resolutions.

The use of the ¹³⁷Cs technique in a Mediterranean environment.

The ¹³⁷Cs technique has been used in all sorts of environments all over the world to estimate net soil redistribution rates. However, its potentials in areas with shallow and stony soils on hard rock lithology remain unclear. Concentrations in the soil of artificial and natural radionuclides are investigated to assess the applicability of this technique in the study area as a mean to estimate soil redistribution (Chapter 4) and to calibrate the LAPSUS model (Chapter 5). The radionuclide concentrations vary in relation to lithology: natural radionuclides such as Potassium-40 ( ⁴⁰K), Uranium-238 ( ²³⁸U) and Thorium-232 ( ²³²Th) show significant higher concentrations in the gneiss than in the serpentinite soils for both reference profiles as all other samples. This as opposed to the artificial radionuclide Caesium-137 ( ¹³⁷Cs), which is found significantly higher in the serpentinite soils, for the reference profiles probably because of the difference in clay mineralogy and for the transect samples because of difference in soil distribution. The exponential decrease of ¹³⁷Cs with soil depth and its homogeneous spatial distribution emphasise the applicability of the ¹³⁷Cs technique in this type of Mediterranean environments. The spatial distribution of the ¹³⁷Cs inventory and concentration are in agreement with the soil erosion and degradation indicators measured in the field. Surfaces with erosion or degradation features (higher bulk density, shallow soils or surface crust development) show lower ¹³⁷Cs concentrations and inventories, while protected surfaces by vegetation show higher ¹³⁷Cs concentrations and inventories. The distribution of ¹³⁷Cs along the slopes can be explained within existing conceptual models. In this way the serpentinite and gneiss slopes are classified in four models according to the present soil redistribution and the detection of erosion and deposition areas.

Furthermore the landscape evolution over the past 37 years is evaluated. Estimating net soil redistribution rates from radionuclide distributions depend on the calculation of the local area reference inventory and the applied calibration technique. The resulting net soil redistribution estimates are compared with simulations of the LAPSUS model. Total net soil loss for the research area ranges from 2 to 69 [t ha ^-1a ^-1] for serpentinite and gneiss slopes respectively. Differences in total slope sediment budgets as well as differences along the transects reveal influences of landscape representation and land use. In this case the impact of tillage erosion is far more important than possible parent material induced differences.

Dynamic landscape, soil and water redistribution.

Soil suitability assessments for land use purposes are commonly based on on-site specific topographic, soil and climatic characteristics, often neglecting the effects of physical landscape processes by water. The LAPSUS model is applied, including the effects of soil and water redistribution within the landscape (run-on, run-off, erosion and sedimentation) on soil water availability (Chapter 6). The approach focuses at the coarser level of multiple catchments over a period of ten years. By means of four scenarios with increasing complexity, patterns of soil loss and sediment deposition are simulated and resultant effects of water routing, soil depth and erodibility on water availability are evaluated. The model operates in the landscape context using annual time steps and both on-site effects (local changes in terms of boundary conditions) and off-site effects (caused by changes elsewhere) are accounted for. Different approaches for surface run-off routing have a major influence on the magnitude and spatial patterns of water and soil redistribution within the landscape, as well as initial conditions such as soil depth, parent material characteristics and erodibility. Locally decreasing water storage capacity (on-site) may cause increased run-off and erosion at lower positions in the landscape (off-site). Apparent acceptable mean regional soil loss rates, often include local soil redistribution rates that cause significant changes in actual soil depth, indirectly affecting related total amounts of available soil water.

Linking landscape process modelling and land use change.

LAPSUS is also used to explore the impacts of land use changes scenarios on landscape development (Chapter 7). For a period of 10 years LAPSUS calculates soil redistribution (erosion and sedimentation) for three scenarios. Main inputs are a DEM, precipitation and land use related infiltration and erodibility. Examples are shown of both on-site as well as off-site effects of land use change and the influence of different pathways of change. Each scenario produces different spatial and temporal patterns of total amounts of erosion and sedimentation throughout the landscape. Consequently, potential land use related parameters like soil depth, infiltration and flooding risk change significantly too. The scenario of an abrupt change produces the highest erosion rates, compared to the gradual change scenario and the baseline scenario. However, because of the multi-dimensional characteristics of the landscape not only the area suffering from land use change is affected. Increasing erosion and run-off rates from upstream-located olive orchards have an impact on downstream local run-on, erosion and sedimentation rates. In this case the citrus orchards situated in the valley bottom locally suffer damages from re-sedimentation events but benefit from the increase in run-on water and nutrients.

Synthesising,

the landscape was studied at different levels of temporal and spatial extensions and resolutions. Consequently it is not easy to link the results of the processes understood at those different levels, however the abstraction of some findings can give some direction: according to the geological evidence in this case study area the final present day uplift rates range between 0.07 to 0.1 [mm a ^-1]. These rates are in the same order of magnitude as the net erosion rates for natural slopes measured with the ¹³⁷Cs technique. This suggests a link of the spatial temporal resolution of geological landscape evolution and actual natural landscape development. At the same time the cultivated areas on gneiss lithology indicate a factor 10 or more increase of soil redistribution, demonstrating the enormous impact of human induced landscape evolution and land use change.

LAPSUS has been developed as a single process landscape evolution model, based on the potential energy content of flowing water over a landscape surface and the continuity equation for sediment movement, operating at the landscape-annual scale. The temporal components of the model are a compromise between the spatial resolution of interest and the applied process based lumped parameters. It also can be used at different grid sizes. It has shown quite reasonable results for simulating erosion/accumulation rates at slope, subcatchment and catchment scale, introducing the effect of different lithologies and land uses. This simplification of the reality and the isolation of the influence of different factors in the landscape evolution can help to understand the on-site and off-site effects of land use changes on the landscape and the impact on the sustainability development of the region.