Supplement of Modelling the effect of catena position and hydrology on soil chemical weathering

Abstract. The sensitivity of chemical weathering to climatic and erosional forcing is well established at regional scales. However, soil formation is known to vary strongly along catenas where topography, hydrology, and vegetation cause differences in soil properties and, possibly, chemical weathering. This study applies the SoilGen model to evaluate the link between the topographic position and hydrology with the chemical weathering of soil profiles on a north–south catena in southern Spain. We simulated soil formation in seven selected locations over a 20 000-year period and compared it against field measurements. There was good agreement between simulated and measured chemical depletion fraction (CDF; R2=0.47). An important variation in CDF values along the catena was observed that is better explained by the hydrological variables than by the
position along the catena alone or by the slope gradient. A positive trend between CDF data and soil moisture and infiltration and a negative trend with water residence time was found. This implies that these hydrological variables are good predictors of the variability in soil properties. The model sensitivity was evaluated with a large precipitation gradient (200–1200 mm yr−1). The model results show an increase in the chemical weathering of the profiles up to a mean annual precipitation value of 800 mm yr−1, after which it drops again. A marked depth gradient was obtained
for CDF up to 800 mm yr−1, and a uniform depth distribution was obtained with precipitation above this threshold. This threshold reflects a change in behaviour, where the higher soil moisture and infiltration lead to shorter water transit times and decreased weathering. Interestingly, this
corroborates similar findings on the relation of other soil properties to precipitation and should be explored in further research.


Colour 7,5YR 4/3; Brown; Sandy clay; structure granular, size very fine to fine; soft; few pores, very fine, irregular; few roots, very fine, distributed throughout the horizon; boundary abrupt and smooth.
a Munsell reading was taken on moist soil. Colour 5YR 4/6; Yellowish Red; Sandy loam; structure granular, size medium to coarse; soil loose; no pores; common roots, very fine to very coarse, distributed throughout the horizon; boundary abrupt and wavy; horizon very weathered, no igneous rocks fragments.
a Munsell reading was taken on moist soil. Colour 7,5YR 2,5/2; Very Dark Brown; Sandy loam; structure granular, size médium to coarse; soft; moderately few pores, very fine to medium, irregular; common roots, very fine to very coarse, distributed throughout the horizon; boundary clear and wavy; 2% igneous rocks fragments, discoidal and angular, gravel to cobbles.
Color 7,5YR 4/3; Brown; loamy sand; structure granular, size medium to very coarse; slightly hard; moderately few pores, very fine to medium, irregular; moderately few roots, very fine to very coarse, distributed throughout the horizon; 20-25% igneous rocks fragments, subidscoidal and angular, gravel to boulders.
a Munsell reading was taken on moist soil. Colour 7,5YR 2,5/2; Very Dark Brown; loamy sand; structure subangular, size fine to medium; soft; moderately few pores, very fine to medium, irregular; common roots, very fine to very coarse, distributed throughout the horizon, boundary diffuse and smooth; 5-10% igneous rocks fragments, discoidal and very angular, gravel.
Colour 7,5YR 4/3; Brown; loamy sand; structure granular, size medium to very coarse; soft; moderately few pores, very fine to medium, irregular; moderately few roots, very fine to coarse, distributed throughout the horizon; boundary gradual and wavy; 15-20% igneous rocks fragments, discoidal and very angular, gravel to stones; two large stones, spherical and subangular, the first (30x25 cm) between 45-60 cm depth, the second (40x22 cm) between 60-83 cm depth. Colour 10YR 5/6; Yellowish Brown; Sandy loam; structure subangular; size medium to coarse; slightly hard; few pores, very fine to very coarse, irregular; common roots, very fine to very coarse, distributed throughout the horizon; boundary abrupt and irregular; 2-5% igneous rocks fragments, discoidal and very angular, medium gravel.
Colour 7,5YR 5/8; Strong Brown; sand; moderately hard; few pores, very fine to very coarse, irregular; common roots, very fine to very coarse, distributed throughout the horizon; slightly weathered igneous rocks fragments, coarse to very coarse, manganese coating between the fragments.
a Munsell reading was taken on moist soil. Colour 7,5YR 4/6; Strong Brown; loamy sand; structure subangular; size very fine to fine; soft; few pores, very fine, irregular; common roots, very fine to coarse, distributed throughout the horizon; boundary abrupt and irregular; 2-5% igneous rocks fragments, discoidal and very angular, fine gravel, Colour 7,5YR 6/8; Reddish Yellow; sand; structure massive; size medium; slightly hard; few pores, very fine, irregular; common roots, very fine to coarse, distributed throughout the horizon; very weathered igneous rocks fragments (parent material).
a Munsell reading was taken on moist soil. Colour 7,5YR 3/4; Dark Brown; loamy sand; structure subangular, size medium to coarse; soft; few pores, very fine, irregular; common roots, very fine to fine, distributed throughout the horizon; boundary very abrupt and wavy; 0-2% igneous rocks fragments, discoidal and very angular, gravel.
Colour 7,5YR 7/8; Reddish Yellow; Sandy clay; structure massive, size medium to coarse; moderately hard; few pores, very fine, irregular; few roots, very fine to coarse, distributed throughout the horizon.
Colour 7,5YR 5/6; Strong Brown; sand; structure massive; unit soil loose; few pores, very fine, irregular; few roots, very fine to coarse, distributed throughout the horizon; boundary gradual and smooth; 0-2% igneous rocks fragments, discoidal and very angular, gravel, clay accumulation surfaces above fragments, accumulation of roots in water channels.