Elsevier

CATENA

Volume 127, April 2015, Pages 80-91
CATENA

Effects of rainfall intensity on groundwater recharge based on simulated rainfall experiments and a groundwater flow model

https://doi.org/10.1016/j.catena.2014.12.014Get rights and content

Highlights

  • Combined mathematical models with artificial rainfall simulation experiments

  • The impacts of rainfall intensities on groundwater regime were studied.

  • Increasing rainfall intensities would decrease recharge coefficients of groundwater.

  • Recharge rate and flow increased first then decreased with intensities increasing.

Abstract

It is important that groundwater discharges sustain baseflow to rivers for the ecological basic flow protection. The objective of this study was to assess the impact of rainfall intensity on groundwater regime under the bare slope condition. A three-dimensional finite-difference groundwater flow model (MODFLOW) was constructed and calibrated and combined with simulated rainfall experiments to study this impact. Groundwater recharge coefficients for different rainfall intensities with a constant amount of rainfall (120 mm) were calculated by using PEST-ASP program of MODFLOW. The values decreased from 0.439 to 0.345, 0.327, 0.167, 0.138, 0.076 with rainfall intensities increasing from 45 mm/h to 60/75/90/105/120 mm/h respectively; recharge coefficients were described by a negative linear relationship. The simulated scenarios indicated decreases in both recharge volumes and the hydraulic head coincided with increases in rainfall intensities, while recharge rates and runoff of groundwater increased with modest intensities (≤ 75 mm/h) increasing and decreased when intensities became larger (> 75 mm/h). Though recharge rate and runoff for different rainfall intensities did not approach a common value, but instead stabilized at different values for each rainfall intensity event. It was concluded that rainfall intensity has great influence on groundwater regime. These are of great importance in ecological basic flow protection, river harnessing and watershed management in some extent.

Introduction

The dynamic changes of climate and human activities have altered the natural flow of rivers (Gädeke et al., 2014, Li et al., 2014, Richter et al., 1997, Sparks, 1992, Xu et al., 2014) and in some arid and semiarid regions, flow of many rivers continue to decrease, even appeared zero flow in dry season (Liu and Chen, 2000; Ren et al., 2002, Wang et al., 2006). Weihe River, located in the arid and semiarid regions of China, is a typical water deficiency and ecological basic flow shortage river, which has brought a great impact to the economic and social development of the region (Lin and Li, 2010, Liu and Hu, 2006). The amount of water infiltrating the soil surface directly affects the quantity of surface runoff and the recharge of both soil and ground water (Liu et al., 2011) which are the main sources of river flow. So the river flow, especially the ecological basic flow which maintained by groundwater during periods of low or no rainfall is affected directly by rainfall infiltration.

The quantity of surface runoff and soil recharge that came from rainfall is highly dependent on rainfall intensity and the relationships between them have been studied in detail (Huang et al., 2012, Jungerius and Ten Harkel, 1994, Schindewolf and Schmidt, 2012, Shigaki et al., 2007). By using a rainfall simulator, an experiment that was conducted in Loess Hilly region showed that rainfall intensity had significant effects on the runoff, and there was a negative exponential relationship between rainfall intensity and runoff coefficient (Li et al., 2014, LI and Huang, 2009, LI and Shao, 2004). The run off–on–out (ROOO) method was used to quantitatively measure the clay loam soil infiltrability under three rainfall intensities (20, 40 and 60 mm/h) and concluded that lower rainfall intensity resulted in higher infiltration rates and greater cumulative infiltration (Liu et al., 2011). Some workers claimed that rainfall intensities had a negative influence on infiltration because raindrop impact destroyed the surface aggregates of soils and gradually formed a continuous crust (Brandt and Thornes, 1987, Dunne and Leopold, 1978, Hoogmoed and Stroosnijder, 1984, Morin and Benyamini, 1977), which has very low hydraulic conductivity. Huang et al. (2012) conducted a quantitative study on soil infiltration and its factors using simulated outdoor rainfall events and found the recharge coefficient decreased with increasing of rainfall intensity. And the covering measures that improve the relationship between rainfall intensity and soil infiltration also have been discussed extensively. For example, when the experiments were conducted under rangeland vegetation–soil associations (the soil was covered with rock, litter, vegetation base, grass, shrub, forb), the results from 19 rainfall simulation runs showed that the increase in infiltration rate with rainfall intensity increased from 0 to 176 mm/h (Stone et al., 2008). These covering measures dispersed the large raindrops into small raindrops, which reduced the actual rainfall intensity on the ground and made the rainfall intensity less than the infiltration capacity.

However, there is a need for more detailed investigations of rainfall intensity effects on groundwater recharge. Recharge results from effective precipitation (that is, precipitation minus losses from evapotranspiration) which infiltrate into the subsurface from where hydraulic gradients are downward (Taylor et al., 2013a). In many environments, natural groundwater discharges sustain baseflow to rivers, lakes and wetlands during periods of low or no rainfall (Taylor et al., 2013b), so increased attention was given to the effect of rainfall on groundwater recharge (Assouline and Mualem, 1997, Foley and Silburn, 2002, Hawke et al., 2006). However the impact of changing rainfall intensities on groundwater recharge remained unclear. Dourte et al. (2012) found that greater intensity storms might reduce groundwater recharge and increase runoff. However, Owor et al. (2009) using a rare set of coincidental observations of daily rainfall and groundwater levels in a seasonally humid equatorial basin (Upper Nile) found that projected increases in rainfall intensities as a result of global warming might promote rather than restrict groundwater recharge. In East Africa, Taylor and Howard (1996) found that rainfall and recharge had a nonlinear relationship and groundwater resources depended on extreme rainfall. Groundwater discharges sustain baseflow of Weihe River, so correctly estimating the groundwater recharge process over time is of importance for Weihe River in ecological basic flow protection, river harnessing and watershed management.

Both monitoring and modeling approaches have been used to measure or estimate groundwater recharge in the literatures. The monitoring approaches could be divided into physical methods and chemical methods (Sophocleous, 1993). The physical methods were (1) hydrometeorologic and soil–crop data processing to determine the soil–water balance or hydrologic balance of an area; (2) hydrologic data interpretation, including water table fluctuation analysis and different streamflow or streamflow separation (baseflow) analysis; and (3) soil–physics-based analysis, including estimation of water fluxes beneath the root zone using unsaturated hydraulic conductivity functions and the gradients in water potential, the zero-flux plane method, and lysimetry; the chemical methods included chemical and isotopic analyses of pore fluids from the saturated and unsaturated zones, with the results significantly affected by the mechanisms of infiltration. Because the monitoring approaches were very highly site-specific, expensive and time demanding, modeling approaches had become a trend and provided estimates of recharge rates over large areas. Fully saturated models, such as a three-dimensional finite-difference groundwater flow model, MODFLOW, were very popular and commonly applied to groundwater recharge problems (Jyrkama et al., 2002). A methodology was developed for linking climate models and MODFLOW to investigate future impacts of climate change on groundwater resources (Scibek and Allen, 2006). Zhang and Hiscock (2010) applied a groundwater flow model (MODFLOW) with a soil moisture balance recharge model to predict the effect of land-use change to forestry on groundwater recharge and levels in Nottinghamshire. Cho et al. (2009) used MODFLOW to determine the impact of land development activities on the subsurface flow regime in the Upper Roanoke River Watershed (URRW). The model interaction between surface water and groundwater also has become a trend, such as an integrated SWAT–MODFLOW was capable of simulating a spatio-temporal distribution of groundwater recharge rates, aquifer evapotranspiration and groundwater levels (Kim et al., 2008). But the accuracy of modeling results depended greatly on the accuracy of the information and the magnitude and distribution of the aquifer permeability (Sanford, 2002).

This paper combined monitoring method with modeling approach to estimate the effects of rainfall intensities on groundwater regime and it could provide a thought for ecological basic flow protection, river harnessing and watershed management.

Section snippets

Experimental conditions and equipment

The simulated rainfall experiments were carried out in the Rainfall Simulation Hall of the State Key Laboratory of Soil Erosion and Dry land Farming on the Loess Plateau in Yangling District, Shaanxi Province, China. The simulated rainfall system has automatic simulation device of under sprinkler and the mean height of fall is about 18 m. The experiments were conducted in the soil box model (Fig. 1), 5.3 m × 1 m × 1 m at the Rainfall Simulation Hall (Wang et al., 2014). It was fitted with a jack to

Parameter calibration of Visual MODFLOW model

The main parameters of the model were: recharge coefficient (Rc), hydraulic conductivity (Kx, Ky, and Kz), storage (specific storage (Ss), and specific yield (Sy)). Initially, based on the simulated rainfall experiment of 75 mm/h rainfall intensity, the model was calibrated manually for the initial set of parameters and subsequently the values of parameters were adjusted accurately using PEST during the optimization process. Calibrated MODFLOW parameters were shown in Table 2.

The scatter graph

Conclusions

Rainfall intensity, which is one of the most important characteristics of rainfall, has an effect on surface runoff and the recharge of both soil and ground water. So it affects the river flow directly, especially the ecological basic flow which is maintained by groundwater during periods of low or no rainfall. Based on the artificial rainfall experiments and groundwater model (Visual MODFLOW), the effects of rainfall intensity on groundwater regime were studied.

Groundwater recharge

Acknowledgments

This paper was supported by the Natural Science Foundation of China (41371276), by the National Technology Support Project (2011BAD31B05) by the Subject of the National Science and Technology Major Project (2009ZX07212-002-003-02), by Knowledge Innovation Project of Institute of Soil and Water Conservation, CAS & MWR (Soil and Water Conservation Project) (A315021304) and (2013KTDZ03-03-01). The author would like to thank Mr. Xiu-quan XU, and Mr. Tong ZHANG for their assistance in experiment

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