**2. Design and construction of artificial hydrological experiments**

Considering the randomness, uncertainty and difficulties of implementing hydrological experiments and relevant studies under natural rainfall condition, the requests of investigating the spatial variability effect of soil characteristics and rainfall characteristics on runoff generation, nutrient fluxes and soil erosion led to the development of rainfall simulators on small plots. Iserloh et al. have mentioned that using small rainfall simulators, among others, has more advantages due to the low costs, easy to transport and use in inaccessible areas and low water consumption [6]. This technique has been used worldwide by different research groups where it was beneficial for the investigation of runoff generation processes, assessments of soil erosion by surface runoff and decision-making in soil and water conservation. The design and constructions of artificial (indoor and field) hydrological experiments using rainfall simulator for different purposes are described below.

#### **2.1. Indoor artificial hydrological experiment**

and soil conservation, prevention of nutrient losses and soil erosion. [1]. Field/pilot rainfallrunoff experiments are traditional and sound approaches to uncover runoff generation processes and assess their responses to changes in topography, land use, soil type, underlying geology and climate patterns. In particular, the artificial rainfall-runoff experiments originated from the 1950s in the USA were conducted for investigation of water and soil conservation and later applied worldwide on research of runoff generation processes and nutrient (nitrogen, phosphorus) losses. Up to now, these approaches are still widely used in investigating nutrient export patterns, the impacts of rainfall pattern (amount, duration and intensity),

Original and reliable data can be obtained from rainfall-runoff experiment and hydrometric monitoring in natural rainfall events. However, it takes long time for conducting rainfallrunoff experiment under natural rainfall condition due to high variability, randomness and uncertainty in rainfall occurrence. It is also heavily impacted by environmental and climatic conditions as rainfall condition cannot be controlled. Therefore, it is difficult to obtain ideal results from runoff generation study under natural rainfall condition and therefore is not efficient. Artificial runoff experiment using rainfall simulator can realise simulation of different rainfall patterns (i.e. combination of different rainfall intensities and different rainfall durations). It can be either conducted in the field or in indoor laboratory by simulating physiographic characteristics and rainfall pattern of the concerned study site. Artificial rainfall simulator can increase the efficiency of the experimental study by overcoming the high randomness in occurrence of diffuse nutrient export and soil erosion under natural rainfall condition [5]. Thus, the artificially rainfall-runoff experiments combined with hydrological and geochemical analysis have become effective techniques for investigation of runoff generation,

nutrient losses and soil erosion processes and estimation of pollutants' export loads.

**2. Design and construction of artificial hydrological experiments**

Considering the randomness, uncertainty and difficulties of implementing hydrological experiments and relevant studies under natural rainfall condition, the requests of investigating the spatial variability effect of soil characteristics and rainfall characteristics on runoff generation, nutrient fluxes and soil erosion led to the development of rainfall simulators on small plots.

ment for water and soil conservation.

186 Hydrology of Artificial and Controlled Experiments

In the following sections, we provide a review on artificial hydrological experiments, with particular focus on those conducted in red soil-covered area in South China, from the perspectives of idea and constructions of artificial experimental facilities, experimental methods (such as hydrometrical methods for various parameters, simulated rainfall equipment, hydrogeochemical methods) and findings obtained from relevant studies. Finally, a summary of challenges in artificial hydrological experiments and outlook of future studies are given. The outcome of this work may provide guidelines for the design and constructions of artificial hydrological experiment based on various objectives, improve understanding of hydrological processes and impacts on nutrient and soil erosion, serve for hydrological modelling development by improving parameter setting and support decision-making on watershed manage-

land use, soil type and antecedent soil water content on nutrient losses [2–4].

**Figure 1** shows the schematic design of indoor artificial rainfall-runoff experiment in a flume, which is used to investigate soil erosion processes and influential factors, including rainfall patterns (e.g. rainfall intensity, duration) and soil characteristics (e.g. soil gradation, porosity, precedent soil moisture). An example of such experiment is shown in **Figure 1**.

The erosion flume has the size of 6 m long × 2 m wide. The flume and sprinkling system are described in detail in [7–10]. A summary of the key features is given here. The flume sits on a hydraulic piston so that the slope can be adjusted. The flume can be filled with 0.32 m of a natural soil taken from field site and is underlain by 0.10 m of coarse gravel to facilitate the

**Figure 1.** Schematic structure of the designed rainfall-runoff soil erosion flume referring to the Ecole Polytechnique Fédérale de Lausanne (EPFL) erosion flume [7, 8]. The slope of the flume can be adjusted within the intervals 0 and 30%. Precipitation is fallen using 10 oscillating sprinkler systems located at 3 m above the soil surface.

excess rainfall drainage. Also, at the downslope end of the flume, there are eight openings collecting the infiltrated water which drains through the soil and gravel. Other four openings (item 2c, **Figure 1**) at the flume's downslope end capture subsurface flow through the topsoil layer. The deep soil and the openings at the bottom and downslope end of the flume allow us to do experiments on unsaturated soils without fully saturating the soil profile during the experiments.

Water is fallen to the flume by 10 VeeJet 80150 nozzles located on an oscillating bar 3 m above the soil surface ensuring the raindrops reached their terminal velocity. The whole flume is divided into two 1-m width identical flumes allowing replicate experiments. Surface runoff is measured as a function of time over the course of the rainfall event for each collector as shown in **Figure 1**. Water from each flume outlet is sampled in individual bottles. Continuous sampling is conducted for the early experiment stage. Afterwards, the sampling time interval increases towards the steady-state equilibrium.

#### **2.2. Field artificial hydrological experiment**

It was recognised that the results obtained under carefully controlled laboratory conditions are rarely directly valid in the field due to the high heterogeneity in terms of the influencing parameters on soil hydrological processes. Thus, additional investigations at the natural field conditions are needed.

The eco-science and technology park of Jiangxi province is an ideal field site for researches on runoff generation, nutrient export and soil erosion [11]. It is located in De'an County of Jiujiang City, Jiangxi province with the total area of 0.8 km2 . For runoff generation study, field soil water leakage experiment plot was constructed with a length of 15 m, width of 5 m (horizontal projection) and slope of 14° . The schematic diagram of the rainfallrunoff experimental plot is shown in **Figure 2**. The four sides and bottom of the plot were constructed using reinforced concrete to isolate from surrounding environment; sandy inverted filter was set on the bottom of the plot. Trapezoidal retaining wall was constructed at the toe of the plot using reinforced concrete. In this way a closed draining soil water infiltration device was built. To prevent water entering the plot, the four sides of the plot were constructed using reinforced concrete, which are 30 cm higher than the land surface. After removal of surficial plant and miscellaneous chips, five layers of undisturbed soil were sampled with thickness of each layer 25 cm, and then each layer was stacked. Mean soil density of each layer was measured at the depth of 10 cm, and then the sampled soil was backfilled in the plot to the depth 105 cm in the order of soil sampling with backfilled depth of 10 cm at each time. The weight of backfilled soil was determined using the formula

$$W = V \cdot \gamma \cdot (1 + S) \tag{1}$$

After backfill of soil at each time, soil was compacted to make it achieve the defined depth of 10 cm in order to make the porosity in the backfilled plot similar as that in the natural condition.

Field-Controlled Hydrological Experiments in Red Soil-Covered Areas (South China): A Review

http://dx.doi.org/10.5772/intechopen.70547

189

**Figure 2.** Schematic diagram of the rainfall-runoff experimental plot adapted from [11].

In the observation room located at the toe of the plot, four water flow outlets are set from top of the retaining wall to the bottom. The upmost outlet is used for collecting surface runoff and sediment. It connects flow collecting barrel at the bottom of the plot to the surface flow pool via PVC tube. The rest of the three outlets collect underground runoff (i.e. interflow of 30 and 60 cm depth and groundwater runoff at 105 cm below the surface). Horizontal flow collecting barrels were installed in the retaining wall vertically at corresponding depth, which were connected to corresponding runoff pool with bottom area of 1.2 m3. Surface flow pool, interflow pool and groundwater flow pool were equipped with self-record water-level sensor (mode HCJ1) to continuously monitor dynamics of runoff and water leakage generated from

Based on intensive literature review, it is found that artificial hydrological experimental studies can be categorised into the following groups with different objectives: (i) runoff and soil erosion processes and response to rainfall pattern, land use type, slope and tillage approach and (ii) nutrient export and response to land use type, slope and rainfall pattern. Now, a sum-

Several parameters are commonly used to describe rainfall-runoff processes, including runoff starting time, share of different runoff components (i.e. surface runoff, interflow at different

different patterns of rainfall events.

**3. Artificial hydrological experimental studies**

**3.1. Impacts of land use and rainfall on runoff processes**

mary of the former studies related to these two perspectives is given.

where *V* is the volume of backfilled soil at each time (m3 ), *γ* is the dry density of natural soil (kg/m3 ) and *S* is the indoor soil water content (%).

Field-Controlled Hydrological Experiments in Red Soil-Covered Areas (South China): A Review http://dx.doi.org/10.5772/intechopen.70547 189

**Figure 2.** Schematic diagram of the rainfall-runoff experimental plot adapted from [11].

excess rainfall drainage. Also, at the downslope end of the flume, there are eight openings collecting the infiltrated water which drains through the soil and gravel. Other four openings (item 2c, **Figure 1**) at the flume's downslope end capture subsurface flow through the topsoil layer. The deep soil and the openings at the bottom and downslope end of the flume allow us to do experiments on unsaturated soils without fully saturating the soil profile during the

Water is fallen to the flume by 10 VeeJet 80150 nozzles located on an oscillating bar 3 m above the soil surface ensuring the raindrops reached their terminal velocity. The whole flume is divided into two 1-m width identical flumes allowing replicate experiments. Surface runoff is measured as a function of time over the course of the rainfall event for each collector as shown in **Figure 1**. Water from each flume outlet is sampled in individual bottles. Continuous sampling is conducted for the early experiment stage. Afterwards, the sampling time interval

It was recognised that the results obtained under carefully controlled laboratory conditions are rarely directly valid in the field due to the high heterogeneity in terms of the influencing parameters on soil hydrological processes. Thus, additional investigations at the natural field

The eco-science and technology park of Jiangxi province is an ideal field site for researches on runoff generation, nutrient export and soil erosion [11]. It is located in De'an County of

field soil water leakage experiment plot was constructed with a length of 15 m, width

runoff experimental plot is shown in **Figure 2**. The four sides and bottom of the plot were constructed using reinforced concrete to isolate from surrounding environment; sandy inverted filter was set on the bottom of the plot. Trapezoidal retaining wall was constructed at the toe of the plot using reinforced concrete. In this way a closed draining soil water infiltration device was built. To prevent water entering the plot, the four sides of the plot were constructed using reinforced concrete, which are 30 cm higher than the land surface. After removal of surficial plant and miscellaneous chips, five layers of undisturbed soil were sampled with thickness of each layer 25 cm, and then each layer was stacked. Mean soil density of each layer was measured at the depth of 10 cm, and then the sampled soil was backfilled in the plot to the depth 105 cm in the order of soil sampling with backfilled depth of 10 cm at each time. The weight of backfilled soil was determined using the

*W* = *V* ⋅ *γ* ⋅ (1 + *S*) (1)

. For runoff generation study,

. The schematic diagram of the rainfall-

), *γ* is the dry density of natural soil

experiments.

188 Hydrology of Artificial and Controlled Experiments

conditions are needed.

formula

(kg/m3

increases towards the steady-state equilibrium.

Jiujiang City, Jiangxi province with the total area of 0.8 km2

of 5 m (horizontal projection) and slope of 14°

where *V* is the volume of backfilled soil at each time (m3

) and *S* is the indoor soil water content (%).

**2.2. Field artificial hydrological experiment**

After backfill of soil at each time, soil was compacted to make it achieve the defined depth of 10 cm in order to make the porosity in the backfilled plot similar as that in the natural condition.

In the observation room located at the toe of the plot, four water flow outlets are set from top of the retaining wall to the bottom. The upmost outlet is used for collecting surface runoff and sediment. It connects flow collecting barrel at the bottom of the plot to the surface flow pool via PVC tube. The rest of the three outlets collect underground runoff (i.e. interflow of 30 and 60 cm depth and groundwater runoff at 105 cm below the surface). Horizontal flow collecting barrels were installed in the retaining wall vertically at corresponding depth, which were connected to corresponding runoff pool with bottom area of 1.2 m3. Surface flow pool, interflow pool and groundwater flow pool were equipped with self-record water-level sensor (mode HCJ1) to continuously monitor dynamics of runoff and water leakage generated from different patterns of rainfall events.
