Effects of land use, land cover and rainfall regimes on the surface runoff and soil loss on karst slopes in southwest China

Abstract

Surface runoff and soil loss from 2007 to 2010 related to land use and rainfall regimes in karst hill slopes in Guizhou Province, southwest China, were analyzed. Using the hierarchical clustering method, sixty-one rainfall events under the subtropical monsoon climate condition were classified into 5 types of rainfall regimes according to the depth, maximum 30-min intensity, and duration of rainfall. In our study, we first demonstrated that the amounts of surface runoff and soil loss on the karst hill slopes were very small compared to the non-karst areas, because the dual hydrological structure in the karst region, including ground and underground drainage systems, could influence the processes of rainfall recharge and runoff generation. Most rainfall water was transported underground through limestone fissures and fractures, while little was in the form of surface runoff. Second, the runoff and soil loss were affected by land use management and vegetation cover. Soil loss was intensified in a descending order to five types of land uses: pastureland > burned area > cropland > combination vegetation land > young forestland. Third, the runoff and soil loss exhibited remarkable variances among different rainfall regimes. Large runoff and soil loss were mainly created by heavy rainfall storms with a rainfall depth of more than 40 mm and a maximum 30-min rainfall intensity of over 30 mm h^− 1. In addition, rainfall storms with large antecedent precipitations could also produce large runoff and soil loss. These observations indicated that limestone fissures and fractures play important roles in surface runoff generation on karst limestone slopes due to their large storage capacity and high infiltration rate. Lastly, the soil erosion risk in the karst pure limestone slope is quite high and should be paid particular attention, especially in regards to over-grazing because the soil loss created by a single heavy rainstorm in pastureland was 5 times the annual soil loss tolerance.

Introduction

Soil erosion has become a severe social and environmental concern throughout the world (Higgitt, 1991, Oldeman, 1994). Land use and land cover (LULC) are two of the most important factors that influence the occurrence and intensity of surface runoff and soil erosion (Kosmas et al., 1997, Morgan, 1995). Proper regulation of LULC can greatly improve soil properties (Kosmas et al., 2000) and reduce soil erosion (Fu et al., 2009, Zhang et al., 2004). Without exception, deforestation and improper regulation have led to severe water and soil loss on mountainsides (Francis and Thornes, 1990, Symeonakis et al., 2007), and have resulted in gully development, which in turn can increase the sediment load in rivers (Garcia-Ruiz and Valero-Garces, 1998, Kasai et al., 2005). Other factors, e.g. rainfall, could also result in raindrop splash and overland flow, and subsequently cause soil erosion (Kinnell, 2005, Nearing and Bradford, 1985, Nearing et al., 1991). Rainfall patterns and regimes play key roles in runoff generation and sediment yield (De Lima and Singh, 2002, Moody and Martin, 2009, Wei et al., 2007, Wei et al., 2009). LULC and rainfall regimes are important factors that control the intensity and frequency of overland flow and surface water erosion. Therefore, understanding the relationship of water and soil loss to rainfall and LULC is important for the management of land use and the conservation of soil and water.

In 1983, a soil loss study was conducted in a karst limestone upland region in western Ireland, which indicated that severe soil loss was caused by forest clearance and human activities (Drew, 1983). Since the beginning of the 1990s, some significant studies on runoff and soil erosion based on statistic and parametric models have been carried out from the perspective of the hydrology of limestone hill slopes in semi-arid or arid areas in the Mediterranean and Australia, using runoff field and simulated rainfall methods (Calvo-Cases et al., 2003, Cerdà, 1997a, Cerdà, 1997b, Cerdà, 1998a, Cerdà, 1998b, Imeson et al., 1998, Kheir et al., 2008, Kosmas et al., 1997). Calvo-Cases et al. (2003) found that limestone slopes behave as a patchwork of runoff and run-on areas. The runoff generation mechanism on limestone slopes could be synthesized into two conceptual models: a Hortonian discontinuous runoff model that takes place in the most degraded slopes or during high intensity rain events, and a mixed runoff generation model where both excess infiltration runoff and excess saturation runoff can happen on the same slope. Runoff on limestone slopes was not continuous along the slope that was generated on degraded surfaces and quickly reinfiltrated in close soil patches with sufficient vegetation cover. However, the water and soil loss in semi-arid or arid limestone areas may not represent conditions in the subtropical humid karst region, and reliable data on water and soil loss is very limited for this type of area (Cao and Yuan, 2005, Ford and Williams, 2007). It has been suggested that karst landform development and its characteristics are determined by lithology, structure, geomography and climate (Williams, 2004). Aridity and extreme cold climate both constrain karst development, particularly, the epikarst where highly weathered carbonate bedrock, exposed at the surface or immediately beneath the surface soil, plays an important role in absorbing, storing and transmitting precipitation (Williams, 2008). Moreover, sub-humid and humid karst areas have a dual hydrological system of ground and underground drainage which can influence the hydrological process on slopes (Williams, 2008).

The Guizhou karst area, which covers ~ 109,084 km² with a population of 37.9 million in Guizhou Province in southwest China, is one of the largest continuous karst areas in the world in the humid climate zone. The karst area constitutes 73% of the total area in Guizhou Province, and 17.42% of the karst landforms are developed on continuously pure limestone (Li et al., 2003, Li et al., 2006). The epikarst is well developed on the limestone bedrock and commonly is 2–5 m thick due to the sub-tropical climate conditions (Jiang et al., 2001). Most of the karst areas, especially limestone area, in Guizhou Province belong to the severe Karst Rocky Desertification (KRD) area (Wang et al., 2004). The soils formed from limestone bedrock are usually 20–40 cm thick on mountaintops and 50–150 mm thick on mountainsides (Chen, 1997). Previous studies have shown that 2000–8000 years would be required to produce 1 cm soil in pure and thick pure limestone areas, because the content of the insoluble residues in pure carbonate rocks is very small, usually less than 5% (Chen, 1997, Feng et al., 2009). Once vegetation is removed, extreme soil loss due to water erosion would occur (Zhang et al., 2011a), and it would be very difficult to regenerate vegetation in the region.

Because of population growth and economic development, many land use practices such as logging, over-grazing and agriculture activities are conducted, especially on hilly land with shallow soil. Approximately 29% of the mountain areas in Guizhou Province have slopes between 17 and 25° and 35% have slopes greater than 25° (Gan et al., 2002). At present, 80% of human activities (grazing and agriculture) in Guizhou Province are focused on slopes greater than 6°. The agricultural land on mountains is 691,800 ha for slopes > 25°, and 281,800 ha for slopes > 35°, which are about 20% and 6%, respectively, of the total dry land in Guizhou Province, respectively (Wang, 2003). While improper land use may cause severe soil erosion and karst rocky desertification in this region, rainfall in the sub-tropical wet monsoon climate could also cause runoff and soil erosion. The sub-tropical wet monsoon climate in humid Guizhou Province is characterized by spatial and temporal seasonal variability and wide yearly rainfall fluctuation. Extreme rainfall events, which are frequent in the rainy season, are very disruptive for fragile karst environments (Zhang et al., 2010).

In the 1990s, several studies suggested that soil erosion was very high in karst areas in southwest China due to the low soil formation rate from the carbonate bedrocks, steep slopes topography, high annual precipitation and poor vegetation cover (Lin and Zhu, 1999). Because of the low soil formation rate from the carbonate bedrocks, the rate of limestone soil formation was considered the soil loss tolerance in karst areas (Cao et al., 2008). The soil loss tolerance of the karst areas in southwest China has been calculated as 30–68 Mg km^− 2 a^− 1 by different groups (Cao et al., 2008, Chai, 1989, Wei, 1995). A few studies have pointed out that, the petrologic assemblage in carbonate areas should be divided into three types which have different soil formation rates according to the amount of argillaceous material in formations. The soil formation rates of the homogenous carbonate rock area, the area of carbonate rock intercalated with clastic rock, and the area of carbonate/clastic rock alternations were 6.84 Mg km^− 2 a^− 1, 45.53 Mg km^− 2 a^− 1, and 103.46 Mg km^− 2 a^− 1, respectively (Li et al., 2006). By monitoring the sediment yields in the main rivers in karst regions in southwest China, the soil erosion rates were found to range from 56 to 1047 Mg km^− 2 a^− 1 (Zhu and Lin, 1995). Using large watershed monitoring data combined with the Geographic Information System (GIS) and the Revised Universal Soil Loss Equation (RUSLE), some other researchers studied soil erosion under different LULC conditions. These studies showed that the annual soil loss in the Maotiao River watershed was between 5 and 80 Mg ha^− 1 year^− 1, with a mean value of 28.7 Mg ha^− 1 year^− 1. The soil loss was closely related with land use, rainfall erosivity and topography (Xu et al., 2008). However, these results of the soil erosion rate in this area exhibited remarkable variations due to the different petrologic assemblages, monitoring methods, and scales. Moreover, since large scale studies can hardly indicate accurate water and soil loss rates at slope scales in the karst system, it is very difficult to quantify the processes of water and soil loss under different rainfall regimes.

In the present study, we investigated surface runoff and soil loss during rainfall events from 2007 to 2010 under 6 different land use conditions at the Puding Karst area (26°15′36″N, 26°15′36″E) of Guizhou Province. We analyzed the effects of land use on soil and water loss on karst limestone hill slopes to elucidate the relationship between surface runoff generation, soil loss on karst limestone hill slopes, and different types of rainfall regimes. We further assessed the soil erosion risk on karst limestone hill slopes in different LULC under the subtropical monsoon climate condition.