QUANTITATIVE ASPECTS OF SOIL FORMATION IN MOUNTAINEOUS AREAS
Abstract:
Andrea Roman Sanchez1, Juan Vicente Giraldez1,2 and Tom Vanwalleghem11 University of Cordoba, Department of Agronomy, Cordoba, Spain (o92rosaa@uco.es).
2 Institute of Sustainable Agriculture. CSIC, Government of Spain Alameda del Obispo s/n, CP 14004 Cordoba, Spain
Abstract: It is well known that soil formation depends on factors such as bedrock, climate, relief, vegetation and time. However, despite the great effort dedicated to these processes, little is known about the quantitative relationship between geomorphological processes and soil formation, especially on long timescales. This study presents a new model of water infiltration and redistribution, temperature and soil processes as a function of properties, such as texture or stoniness, topographical variables, like aspect, climate and vegetation. We have generated daily precipitation and temperature for a 15.000 year period, based on paleoclimatic reconstructions and apply this model on different soil profiles along a catena. This model allows to compare for different topographical positions the importance of water flows through the soil, erosion and soil formation and explain in a simple way their interaction and their relative importance of geomorphological processes on soil formation in landscapes over long time scales.
Key words:soil formation, erosion, model, geomorphology, climate.
INTRODUCTION
Soil formation depends on several factors, of which hydrology and relief control strongly the spatial distribution of soil patterns within a catchment. This work presents some preliminary results of a larger study on soil formation processes in the Sierra Morena area of SW Spain. Figure 1 shows the location of the study area and a typical catena. These soil profiles formed on partially decomposed granites and show considerable differences in development stage. At present, available soil formation models range from simple mass-balance models, for example MILES3D (Vanwalleghem et al., 2013) to more complex models that include soil water movement and chemical processes, for example Soilgen (Finke et al., 2008). While the latter have proven successful for modelling soil profile formation, they are computationally expensive and therefore difficult to apply at the landscape scale. The former however, does not include a representation of soil water and chemistry.
This study therefore proposes to develop a long-term soil evolution model based on a simple soil water balance model. In order to run this soil hydrology model, temperature and rainfall data at a daily time step is needed. Such palaeoclimate data is generally only available at the yearly or seasonal time scale.
The objectives are (i) to analyse and generate daily palaeoclimate input data for the last 25 000 years (ii) to evaluate trends in soil moisture and deep percolation and relate these to soil formation along a catena.
Figure 1. Location of the study area and catena
MATERIALS AND METHODS
Calibration with present-day data
Present-day instrumental records from the Cordoba weather station (1959-2011) from AEMET were used for the calibration of the weather generator.
Generation of palaeoclimate data
Originally, a random Monte Carlo approach was tested for generating temperatura and rainfall. Based on their respective probability function as suggested by Laio et al. (2001). However, problems with phase distortion between rainfall and temperature resulted in the adoption of a Markovian scheme as suggested by Matalas (1967) and modeled by Richardson in the model WGEN. Applicability of this model has been demonstrated by several authors, for example Semenov et al. (1998). The palaeoclimate reconstruction of daily temperature and rainfall was done base on WGEN, calibrated for Cordoba, and combined with rainfall and temperature anomalies presented by Combourieu Nebout et al. (2009) for a period of 25 000 years based on marine pollen records.
Estimation of potential evapotranspiration
To complete the water budget, data is needed on evapotranspiration in the study area. The model of Hargreaves and Samani, (1985) was used.Potential evapotranspiration follows a sine trend
Soil water balance model
The evolution of soil moisture is described with a simple model that combines the models of Thornthwaite and Mather (Steenhuis y van der Molen, 1986) and Brocca y col. (2008), based on a water balance that includes infiltration,f(t),(5) as a function of the ratio between the actual soil moisture,W(t),(3)and its maximum value,Wmax or cc, wetness, with an exponent m that indicates the non-linearity of the process; actual evaporation is the potential evapotranspiration -estimated by Hargreaves (1985)- multiplied by a correction coefficient,cu, which is limited by the amount of available water (1); and finally the deep percolation,g(t),(4)is calculated asuming a unit hydraulic gradient, as the hydraulic conductivity,ks, for the soil moisture at the bottom of the root zone, using the Brooks and Corey function to determine its value.
According to Thornthwaite and Mather, if one considers that evapotranspiration is larger than precipitation, the soil moisture,W(t),can be estimated based on the water loss suffered by the soil,Ph(t)(2)
To include the effect of the relief on the profiles along the catena, (1) plateau, (2) hillslope and (3) valley bottom, the profiles are linked by transferring runoff(q), from the higher to the lower-lying profiles as run-on.
RESULTS
Water balance
Figure 2 shows the water balance in one of the three profiles (1: plateau).
Figure 2 shows the relation between stored soil moisture,W(t), runoff (q), precipitation (p) and evapotranspiration (ev) for one model year.
With respect to the stored soil moisture, it can be observed that the profile reaches maximum soil moisture (of 100 mm) during the winter months. There is a strong seasonal trend, as can be expected in Mediterranean soils, with minimum values during summer.
During the winter rainy season, major runoff events occur. These rains, together with the lower evapotranspiration result in a result in more percolation (Figure 3).
Figure 3 shows the accumulated percolation along one model year in all 3 profiles along the catena.
There is a relationship between the depth of each profile and percolation volume. The plateau profile is more developed than the hillslope and valley bottom profile and it has less depth. This study is intended to adjust these parameters.
Conclusions
Simple model works well to simulate seasonal water balance fluctuations, which are thought critical controls on soil formation in Mediterranean areas. Future work will now focus on introduce the parameters that have influence on the soil formation.
Acknowledgements
This study was funded by the research project AGL2012-40128-C03-02.
Andrea Roman is funded by predoctoral Fellowship Programme, Spanish Ministry of Science.
Tom Vanwalleghem acknowledges funding by the Ramon y Cajal Fellowship Programme, Spanish Ministry of Science
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