# Effects of Soil and Water Conservation Measures on Groundwater Levels and Recharge

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Experimental Conditions and Equipment of Simulated Rainfall Experiments

^{2}, and its coverage was approximately 85%–90%. The third scenario was grassland, the grass species was Ophiopogon japonicus with coverage of 65%–70%, the planting structure was 10-cm row spacing × 5-cm plant spacing × 8.2-cm average plant height and the grass was planted outside 2 months before experiments and transplanted into flume.

**Figure 2.**Pictures of different soil and water conservation measures: (

**a**) Bare Slope; (

**b**) Grass Land; (

**c**) Straw Mulching.

#### 2.2. Experimental Materials and Monitoring Methods

^{3}.

#### 2.3. MODFLOW

^{−4}square meters. The entire model structure was a matrix of 50 rows × 265 columns × 3 layers. The model was divided into three aquifers with gradient of 3°. The thicknesses of the aquifers were 0.5, 1 and 98.5 cm from top to bottom. The edge of the model domain was modeled as a no-flow boundary. The actual quantities of groundwater abstraction from the drain pipe were treated as a flux boundary condition in the form of a pumping well. Groundwater recharge from rainfall was modeled as a recharge package in MODFLOW [22]. According to the actual condition of the rainfall experiments, the groundwater system could be described as a conceptual hydrologic model of three layers, homogeneous, horizontal isotropy, three-dimensional, and transient flow system. The initial conditions refer to the head distribution everywhere in the system at the beginning of the simulation and are thus boundary conditions in time [26]. The initial value and the range of hydraulic conductivity and storage coefficient values at different layers were assigned to each active grid cell by the interpolation of discrete property data that were derived from water releasing test analysis and regional geology data. Combining the initial and boundary conditions, the numeric model was constructed [27].

## 3. Results and Discussion

#### 3.1. Model Calibration and the Key Parameters for Different Measures of Soil and Water Conservation

**Figure 3.**Scatter graphs of the calculated heads vs. observed values for the whole period and t = 70 min during the calibration period (

**a**,

**c**) and the verification period (

**b**,

**d**). The groundwater heads are shown as dots and the solid lines mean the calculation heads are equal to the observed levels.

**Figure 4.**Time-series graphs of the calculated vs. observed values during the calibration period (

**a**) and the verification period (

**b**). The observed heads are shown as larger dots and triangles. The calculated heads are shown as smaller dots and triangles with lines.

**Figure 5.**The scatter graph of the calculated vs. observed values based on the calibrated parameters for different underlying surface: (

**a**) Grass Land; (

**b**) Straw Mulching. The groundwater heads are shown as dots and the solid lines mean the calculation heads are equal to the observed levels.

The Key Parameters | α (10^{−5} m/s) | Sy1 | Sy2 | Sy3 |
---|---|---|---|---|

Bare Slope | 0.68 | 0.36 | 0.36 | 0.36 |

Grassland | 1.72 | 0.22 | 0.22 | 0.27 |

Straw Mulching | 1.92 | 0.072 | 0.072 | 0.26 |

#### 3.2. Model Verification for Different Measures of Soil and Water Conservation

#### 3.2.1. Verification of the Groundwater Balance

^{3}which increased the water storage by 0.3871 m

^{3}. The total volume flow out of the model was 0.9114 m

^{3}, which decreased the water storage by 0.3624 m

^{3}and discharged 0.5490 m

^{3}of recharge water. The mass balance error for the simulation inflow and outflow was 0.03%.

^{3}, which increased the water storage by 0.4331 m

^{3}. The total volume flow out of the model was 1.0196 m

^{3}, which decreased the water storage by 0.4596 m

^{3}and discharged 0.5600 m

^{3}of the recharge water. The mass balance error for the simulation inflow and outflow was 0.13%. For the bare slope scenario, the mass balance error was 0.22% [26]. The results of the simulation may generally be considered acceptable, provided that the models were also calibrated [25].

**Figure 6.**The flow mass balance graphs of the bare slope (

**a**); grassland (

**b**) and straw mulching scenarios (

**c**).

#### 3.2.2. Verification of the Groundwater Level

#### 3.2.3. Verification of the Groundwater Runoff

**Figure 7.**The graphs of the calculated vs. observed heads for three scenarios: (

**a**) Bare Slope; (

**b**) Grass Land; (

**c**) Straw Mulching; (

**A**) The scatter graphs; (

**B**) Time-series graphs.

The Evaluation Indexes | RM/m | ARM/m | SEE/m | RMS/m | NRMS/% | Cor |
---|---|---|---|---|---|---|

Bare Slope | 0.0002 | 0.007 | 0.001 | 0.010 | 4.146 | 0.996 |

Grassland | 0.012 | 0.021 | 0.003 | 0.025 | 8.336 | 0.981 |

Straw Mulching | 0.018 | 0.028 | 0.004 | 0.035 | 9.932 | 0.983 |

**Figure 8.**The scatter graph of the calculated vs. observed groundwater runoff for three scenario: (

**a**) Bare Slope; (

**b**) Grass Land; (

**c**) Straw Mulching.

#### 3.2.4. Verification of the Groundwater Flow Field

**Figure 9.**Flow field of the groundwater at 100 min for three scenarios: (

**a**) Bare Slope; (

**b**) Grass Land; (

**c**) Straw Mulching (The direction of reference vectors stands for flow direction of groundwater and the size of reference vectors only means high or low velocity of flow, not the exact value of flow rate).

#### 3.3. The Impact of Soil and Water Conservation Measures Construction on Groundwater

#### 3.3.1. Response of Groundwater Recharge and Runoff

^{−4}m

^{3}/m/min for the bare slope scenario to 25.53 × 10

^{−4}m

^{3}/m/min for the straw mulching scenario, whereas the corresponding values of the model average difference were 8.29 × 10

^{−4}m

^{3}/m/min and 15.51 × 10

^{−4}m

^{3}/m/min, respectively. Similarly, compared to that of the bare slope scenario, the maximum and average increase in the grassland scenario were smaller than those in the straw mulching scenario. In the time domain, there were significant differences in the magnitude of discharge among the three scenarios with a constant amount of rainfall (120 mm), but all of the time-series lines of groundwater runoff (Figure 11) exhibited similar trends in which the groundwater runoff initially increased sharply with the rainfall but then decreased gradually after its termination. The increased average groundwater runoff during the simulated period for the straw mulching scenario was more than twice as large as that for the grassland scenario compared to that of the bare slope scenario. Table 3 summarizes the changes in the average and maximum values and percent change in the groundwater flow among the three scenarios. The groundwater recharge very strongly depended on the land-use type [33].

Scenario | Runoff | ||
---|---|---|---|

Value (10^{−4} m^{3}/m/min) | Change (%) | ||

Average | Bare Slope | 8.29 | 0 |

Grassland | 10.19 | 22.9 | |

Straw Mulching | 15.51 | 87.1 | |

Maximum | Bare Slope | 10.81 | 0 |

Grassland | 14.82 | 37.1 | |

Straw Mulching | 25.53 | 136.2 |

#### 3.3.2. Response of the Groundwater Level

**Figure 12.**Comparison of the magnitude of the groundwater level fluctuations arisen from land cover variations (t = 0–120 min).

**Figure 13.**Comparison of the magnitude of the groundwater level fluctuations after termination of the rainfall (t = 120 min).

**Figure 14.**Flow field of the groundwater after termination of the rainfall for three scenarios: (

**a**) Bare Slope; (

**b**) Grass Land; (

**c**) Straw Mulching.

**Figure 15.**Time-series graphs of the groundwater level fluctuations among three scenarios during the simulated period.

**Figure 16.**Time-series graphs of the groundwater level fluctuations among three scenarios during the simulated period.

#### 3.3.3. Response of the Stage-Discharge Relationship of the Groundwater

^{−4}m

^{3}/m to 9.5 × 10

^{−4}m

^{3}/m, and the mean level increased from 55.8 to 59.4 cm with a mean value of 57.7 cm. Comparatively, for the grassland and straw mulching scenarios, the discharge increased approximately 3.2 and 2.1 times, respectively, during the rainfall.

**Figure 18.**Stage-discharge relationship of the groundwater for three scenarios during rainfall (t = 0–120 min).

**Figure 19.**Stage–discharge relationship of the groundwater for three scenarios during rainfall (t = 0–120 min).

#### 3.4. The Impact of Soil and Water Conservation Measures Destruction on Groundwater

^{3}and 0.261 m

^{3}in the straw mulching and grassland scenarios, respectively, whereas the volume was only 0.159 m

^{3}in the observed bare slope scenario. Therefore, when the land-use converted from straw mulching and grassland to bare slope, the volume of the groundwater recharge decreased by 42.2% and 39.1%, respectively.

**Figure 21.**The scatter graph of the calculated vs. observed values when the underlying surface converted from straw mulching and grassland to bare slope. (

**a**) shows the compared results when the underlying surface converted from straw mulching to bare slope; (

**b**) shows the compared results when the underlying surface converted from grassland to bare slope. The groundwater heads are shown as dots and the solid lines mean the calculation heads are equal to the observed levels.

## 4. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**MDPI and ACS Style**

Wang, H.; Gao, J.; Li, X.; Wang, H.; Zhang, Y.
Effects of Soil and Water Conservation Measures on Groundwater Levels and Recharge. *Water* **2014**, *6*, 3783-3806.
https://doi.org/10.3390/w6123783

**AMA Style**

Wang H, Gao J, Li X, Wang H, Zhang Y.
Effects of Soil and Water Conservation Measures on Groundwater Levels and Recharge. *Water*. 2014; 6(12):3783-3806.
https://doi.org/10.3390/w6123783

**Chicago/Turabian Style**

Wang, Hong, Jianen Gao, Xinghua Li, Hongjie Wang, and Yuanxing Zhang.
2014. "Effects of Soil and Water Conservation Measures on Groundwater Levels and Recharge" *Water* 6, no. 12: 3783-3806.
https://doi.org/10.3390/w6123783