Flood Effect on Groundwater Recharge on a Typical Silt Loam Soil

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Article Flood Effect on Groundwater Recharge on a Typical Silt Loam Soil

Guohua Zhang 1, Gary Feng 2, Xinhu Li 3,*, Congbao Xie 1 and Xiaoyu Pi 4

1 China Irrigation and Drainage Development Center, Beijing 210054, China; [email protected] (G.Z.); [email protected] (C.X.) 2 USDA-Agricultural Research Service, Starkville, MS 39762, USA; [email protected] 3 State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, Xinjiang, China 4 The Water Service Bureau of Chaoyang of Beijing City, Beijing 100026, China; [email protected] * Correspondence: [email protected]; Tel.: +86-099-1788-5428

Received: 30 March 2017; Accepted: 10 July 2017; Published: 14 July 2017

Abstract: Floods are of great concern as the global climate changes, and investigations of flood water infiltration and groundwater recharge are important for water resource management worldwide, especially under conditions of global climate changes. However, information on the relationship between the flood water and groundwater recharge is limited. The objective of this study was to determine the relationship between the flood water depth and the height of groundwater rise using lysimeters and numerical modeling in the floodplain of the Tarim River in northwestern China. The experimental results suggested that the rise in height of the groundwater table was closely related to the flood water ponding depth, and the groundwater depth decreased quickly after flooding due to the high infiltration rate of water originating at the Tarim River. The water table falling velocity was significantly less than the water table rising velocity. If the initial groundwater table was deeper, the variation in the water table rise depth was smaller and the water table falling velocity was slower. The numerical simulation results showed good agreement with the observed data, with a determination coefficient (R2) of 0.87 and a root mean square error (RMSE) of 63.91 cm. A good 2 relationship (R = 0.789) between the initial groundwater table depth (H0), initial soil water content (W0), flood water depth (h), and height of the water table rise (H) was established. Considering that natural and artificial flood frequencies are related to flood time interval (dt), a relationship (R2 = 0.892) was developed between them. These results can enhance the understanding of flood recharge characteristics in the floodplains of inland rivers.

Keywords: arid land; ﬂood; groundwater recharge; inland river; lysimeter

1. Introduction Floods are important sources of groundwater recharge in most of the world’s arid lands [1]. With appropriate management practices, floods can benefit the ecology of arid and semi-arid areas [2]. Flood spreading (due to river overflows) is one of the methods used for flood management and water harvesting that increases the groundwater recharge [3]. Recharge is the portion of infiltration that reaches the water table after passing through the soil profile [4]. In arid environments, stream beds of ephemeral rivers are largely composed of permeable, coarse alluvial sediments that promote relatively rapid infiltration of flood water, which then recharges the local alluvial aquifers [5]. Several methods including direct observation [5,6], mathematical modeling [7], and stable isotope studies [8,9] have been used to estimate flood water infiltration and groundwater recharge. Groundwater is an important source of water for the growth of natural vegetation in arid and semi-arid areas [10]. As drought, climate change, and increasing potable water demands from an

Water 2017, 9, 523; doi:10.3390/w9070523 www.mdpi.com/journal/water Water 2017, 9, 523 2 of 15 increasing population take their toll on water resources, better management and ways to store and recharge groundwater are paramount, and underground water storage could be a much-needed solution [11]. Groundwater depth is a critical feedback variable that determines vegetation dynamics [12]. Many studies have focused on the natural vegetation responses to groundwater change [13,14] as a result of artificial watering in arid areas. Change in the groundwater table can influence the development and composition of vegetation, which in turn affects the fragile ecosystem [15,16]. Therefore, the successful conservation of riparian forests will require knowledge of groundwater and stream flow [17]. Direct infiltration and groundwater recharge in arid environments are relatively low due to the rarity of rainstorms, low mean average precipitation, and high potential evaporation; therefore, in many desert areas, direct rain infiltration is regarded as nonexistent [18]. Water resource management in arid environments currently rely on quantifying flood water infiltration and percolation, which recharges the shallow alluvial aquifers [5]. Flood water infiltrates ephemeral channels, recharging local and regional aquifers, and is the main water source in hyper-arid regions [1], while also being an important water source for mountain-front recharge and interbasin flow [19,20]. Furthermore, understanding and estimating the recharge from overbank floods is important [4] as the infiltrated flood water provides the water resources necessary for maintaining human settlements, riparian vegetation, and wildlife along the rivers. Therefore, an understanding of floods and their associated recharge of alluvial aquifers is crucial for better management of valuable water resources [1]. It is also important to characterize the flood by quantifying the water volume involved in the event and not only at the height of the flood. This can be done by measuring the flood discharge during, after, and before the event, as mentioned by Tazioli [6]. Managed aquifer recharge, a more scientific term for the process of purposefully refilling aquifers with surface water to more effectively manage water supply, is also being studied in Arizona using 19 recharge basins as part of the Tonopah Desert Recharge Project, where researchers allowed surface water to slowly refill the underlying aquifer and carefully monitored wells to measure its progress [11]. The episodic recharge of aquifers during overbank floods is one component of the water balance, and is distinct from bank storage [21–23] or river losses, but involves vertical infiltration from overbank flows. Long-term development and management of groundwater in arid regions, especially in alluvial aquifers, depends on the establishment of a balance between the amounts of recharge and withdrawal. To accomplish this goal, recharge components must be carefully assessed [24]. The flood water infiltration process and its effect on groundwater recharge were measured in field experiments and simulated in previous studies [1,5,25]. As the volume of recharge during flooding is highly variable between catchments, it is difficult to quantify in the field [26]. Lysimeters play an important role in the understanding of soil water dynamics and allow a better understanding of the role of soil in the regional water balance [27]. Therefore, a study to investigate the flood effect on groundwater using lysimeters may provide a better understanding of the dynamics of flood water recharge of groundwater. The relationship between hydraulic conductivity and pressure head is highly non-linear, and the direct measurement of these properties in the laboratory, as well as in the field, is often time consuming and expensive [28]. Therefore, this study applied the Richards equation model to simulate the process of groundwater flood recharge. In addition, the numerical model that was applied also assisted in providing a better understanding of the processes regarding the flood water recharge of groundwater. The Tarim River is the largest continental river in China. More than 90% of the existing floodplain forests along the Tarim River rely on floods [29], and these forests provide a wide range of ecosystem services and functions. Groundwater is the main water source to maintain the regeneration of natural vegetation [30–32]. Planned or controlled flooding is an effective method for rehabilitating rivers and reversing ecosystem degradation [33]. Historically, the Tarim River was a mobile stream before the new embankment project was established, and was dominated by trees of Populus euphratica that mainly relied on groundwater and flood water to survive and grow. However, a new embankment project reduced the natural river flooding of the Tarim River. To recover the degenerated natural vegetation, Water 2017, 9, 523 3 of 15

ecologicalWater 2017 water, 9, 523 consumption was artificially regulated by an ecological gate in the Tarim River3 of [1534 ,35]. The purpose of the ecological gate was to provide river water (similar to natural floods before) to plants alongrecover the Tarim the degenerated River. Ecological natural gates vegetation, controlled ecological the river water flooding consumption of the Tarimwas artificially River. Artificial regulated floods haveby been an usedecological in many gate regulatedin the Tarim streams River and[34,35]. rivers The to purpose maintain of the plant ecologic communitiesal gate was [36 to]. Theprovide planned overflowriver playswater an(similar important to natural rolein floods the maintenance before) to plants of plant along cover the onTarim the River. land adjacentEcological to gates the Tarim controlled the river flooding of the Tarim River. Artificial floods have been used in many regulated River. Therefore, it was necessary to investigate the flood effect on the groundwater in floodplain soils. streams and rivers to maintain plant communities [36]. The planned overflow plays an important Existing literature [12–14] has found that a groundwater depth required to maintain growing role in the maintenance of plant cover on the land adjacent to the Tarim River. Therefore, it was vegetationnecessary should to investigate be less than the six flood meters effect in on the the low groundwater reaches of thein floodplain Tarim River. soils. However, the groundwater level is usuallyExisting deeper literature than [12–14] six meters, has found therefore that impeding a groundwater local vegetationdepth required growth to maintain [37]. Previous growing studies havevegetation investigated should the effectbe less of than the Tarimsix meters Basin in overflowthe low reaches on the of ecological the Tarim characteristics River. However, of riparianthe vegetationgroundwater [10,38 ,level39]. Liis usually et al. [35 deeper] found than that six floodmeters, water therefore could impeding percolate local into vegetation the floodplain growth soils due to[37]. a highPrevious infiltration studies have rate. investigated Other studies the describedeffect of the in Tarim References Basin overflow [10,13,40 on] reported the ecological that river floodscharacteristics contributed of to riparian a rise in vegetation the groundwater [10,38,39]. levelLi et al. in [35] the floodplainsfound that flood of the water Tarim could River; percolate however, the processinto the of floodplain a flood affecting soils due the to groundwater a high infiltration was not rate. clear. Other Previous studies studies described have in alsoReferences shown that [10,13,40] reported that river floods contributed to a rise in the groundwater level in the floodplains flooding could raise the groundwater table in low reaches of the Tarim River [31]; however, information of the Tarim River; however, the process of a flood affecting the groundwater was not clear. on the relationship between the flood water depth and height of the groundwater rise is limited, where Previous studies have also shown that flooding could raise the groundwater table in low reaches of therethe is a Tarim lack of River measurements [31]; however, and information data about on the th relationshipe relationship between between thethe groundwaterflood water depth level and and the floodheight height of is the unknown. groundwater As mentionedrise is limited, above, where whether there is ora lack not of the measurements height of groundwater and data about rises the due to floodrelationship recharge can between reach the an optimalgroundwater groundwater level and the table flood depth height that is couldunknown. maintain As mentioned natural above, vegetation survivalwhether [41] isor of not great the concernheight of in thegroundwater inland rivers rises ofdue arid to regions. flood recharge can reach an optimal Determininggroundwater table the relationshipdepth that could between maintain the natural flood ponding vegetation depth survival and [41] change is of great in the concern groundwater in tablethe in theinland river rivers floodplain of arid regions. of the Tarim River is critical in assessing the contribution of natural or artificial plannedDetermining overflow the relationship to groundwater between recharge. the flood Thisponding study depth aims and to change (1) investigate in the groundwater the flood effect table in the river floodplain of the Tarim River is critical in assessing the contribution of natural or on groundwater using lysimeters and a numerical model; and (2) determine the relationship between artificial planned overflow to groundwater recharge. This study aims to (1) investigate the flood the flood water depth and height of the groundwater rise. effect on groundwater using lysimeters and a numerical model; and (2) determine the relationship between the flood water depth and height of the groundwater rise. 2. Materials and Methods 2. Materials and Methods 2.1. Study Area The2.1. Study study Area was conducted at the Akesu National Water Balance Station (40.37◦ N, 80.45◦ E, 1028.0 m a.s.l.),The study located was inconducted the upper at reach the Akesu of the National Tarim Basin Water in Balance the Xinjiang Station Province (40.37° N, of northwestern80.45° E, China1028.0 (Figure m 1a.s.l.),). Mountains located in to the north,upper west,reach andof the east Tarim encompass Basin in the the Tarim Xinjiang Basin, Province and the of vast expansenorthwestern of the Taklimakan China (Figure Desert 1). Mountains dominates to much the no ofrth, the west, Basin. and east encompass the Tarim Basin, and the vast expanse of the Taklimakan Desert dominates much of the Basin.

FigureFigure 1. 1.The The schematic schematic map in in the the Tarim Tarim River River basin. basin.

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The average annual precipitation ranges ranges from from 200 200 mm to 500 mm in the mountainous areas, from 50 toto 8080 mmmm onon thethe sideside ofof thethe basin,basin, andand onlyonly 1010 mmmm inin thethe centralcentral basin.basin. Precipitation increases from the west to thethe easteast inin thethe plainplain area,area, andand thethe temporaltemporal distributiondistribution of precipitation throughout the the year year is is strongly strongly heterogeneous. heterogeneous. More More than than 80% 80% of the of thetotal total annual annual precipitation precipitation falls fallsbetween between May Mayand October and October in the in high the highflow flowseason season,, and less and than less 20% than of 20% the of total the occurs total occurs from fromNovember November to the to thefollowing following April April [42]. [42 ].Precipit Precipitationation and and temperature temperature have have been been increasing persistently since the 1980s and si significantgnificant increases in precipitation and temperature were observed 10 3 after the 1990s [[43].43]. The average annual runoff atat thethe inletinlet isis 3.983.98 ×× 1010 mm3 fromfrom 1957 1957 to to 2005 with snowmelt and precipitation in the surrounding high high mountains mountains as as the the main main sources sources of of water water [44]. [44]. The groundwater depth depth was was 2.53 2.53 to to 2.06 2.06 m m from from 1958 1958 to to2000 2000 in the in theupper upper reaches, reaches, and and6.0 m 6.0 to m 9.87 to 9.87m in mthe in lower the lower reaches reaches of the of Tarim the Tarim River River [45]. [The45]. population The population of this of area this areawas wasapproximately approximately 8.73 8.73million million in 2000 in 2000 [46], [46 and], and the the major major land land uses uses were were grassland, grassland, unused unused land land and and wetland, wetland, woodlands woodlands and croplands croplands [47–49]. [47–49]. In Inthe the arid arid desert desert climate, climate, desert desert shrubs shrubs and semi-shrubs and semi-shrubs are dominant are dominant while whilelarge herbaceous large herbaceous plants, plants, shrubs shrubs and trees and grow trees growon the on floodplains the floodplains and low and terraces low terraces alongalong both bothriversides riversides [50]. Woodlands [50]. Woodlands are dominated are dominated by Populus by Populus euphratica, euphratica, and major and shrubs major include shrubs Tamarix include Tamarixspp., Lyciumspp., Lyciumruthenicum ruthenicum, Halimodendron, Halimodendron halodendron halodendron, etc.,, etc.,and and herbs herbs are are mainly mainly composed composed of Phragmites australis,, Apocynum venetum,, AlhagiAlhagi sparsifoliasparsifolia,, KareliniaKarelinia caspicacaspica,, GlycyrrhizaGlycyrrhiza inflatainflata,, etc.etc. 2.2. Experiment Design 2.2. Experiment Design The lysimeters were built at the Akesu National Water Balance Experiment Station. Three lysimeters The lysimeters were built at the Akesu National Water Balance Experiment Station. Three (each 960 mm in diameter and 3700 mm in height) were separated by the observation well (1200 mm lysimeters (each 960 mm in diameter and 3700 mm in height) were separated by the observation well diameter and 3700 mm depth). The lysimeter casings were made of fiberglass reinforced plastic with a (1200 mm diameter and 3700 mm depth). The lysimeter casings were made of fiberglass reinforced wall thickness of 12 mm (Figure2), and each column was a lysimeter. plastic with a wall thickness of 12 mm (Figure 2), and each column was a lysimeter.

Figure 2. The lysimeters used in our experiment at the Aksu National Water Balance Station, Figure 2. Xinjiang, China.The lysimeters used in our experiment at the Aksu National Water Balance Station, Xinjiang, China. The soil column was 3200 mm in height, and a filtering layer (200 mm) underlain by gravel was placedThe immediately soil column beneath was 3200 the mm water in height, table at and the a bo filteringttom of layer each (200lysimeter mm) underlainto prevent by fine gravel particles was placedof soil from immediately flowing out beneath during the drainage, water table then at the the soil bottom was ofpacked each lysimeterwith bulk todensity prevent of 1.54 fine g particles cm−1 to of3200 soil mm from from flowing the surface out during of the drainage, gravel layer, then which the soil left was 300 packed mm from with the bulk soil density surface of to 1.54 the g top cm −of1 tothe 3200 lysimeter mm from for ponding the surface water. of theNo gravelplants layer,were allo whichwed left to grow 300 mm at the from surfaces the soil of surface the lysimeters. to the top A ofporous the lysimeter pipe (20 formm ponding in diameter) water. was No plantsinstalled were in allowedthe layer to of grow gravel at the which surfaces connected of the lysimeters.the water Asupply porous (Figure pipe (203). The mm polymethyl in diameter) methacrylate was installed container in the layer served of gravelas a leveling which container connected to the obtain water a supplyconstant (Figure water3 ).table The before polymethyl irrigation. methacrylate The leve containerling container served could as a levelingbe moved container up and to down obtain by a constantmeans of water a pulley, table which before was irrigation. fixed with The a leveling hand cran containerk. Thus, could the water be moved table upin andthe soil down column by means was ofadjusted a pulley, in which correspondence was fixed with to fluctuations a hand crank. of Thus, the groundwater the water table table in the of soilthe columnnatural wassurroundings. adjusted in correspondenceA neutron probe to tube fluctuations made of of aluminum the groundwater with a tablediameter of the of natural 45 mm surroundings. was installed Ain neutron the center probe of tubeeach madesoil column. of aluminum with a diameter of 45 mm was installed in the center of each soil column.

Water 2017, 9, 523 5 of 15 Water 2017, 9, 523 5 of 15

FigureFigure 3. A 3. diagram A diagram of of the the lysimeters lysimeters withwith the capa capabilitybility to to regulate regulate the the groundwater groundwater table. table.

The leveling container had a water outlet for collecting excess water in a percolation container Thethat levelingreached the container bottomhad of the a water lysimeter outlet due for to collecting water table excess recharge. water The in a water percolation table in container the soil that reachedcylinder the bottomand a leveling of the lysimetercontainer duewere tomaintained water table at recharge.the same level The almost water tablesimultaneously. in the soil The cylinder and agroundwater leveling container control operation were maintained was similar at to the that same described level almostby Schendel simultaneously. [51]. The leveling The container groundwater controlonly operation adjusted wasthe initial similar groundwater to that described table and by was Schendel closed [by51 ].a Thethree-way leveling valve container that was only opened adjusted the initialbefore groundwater the flood experiments. table and The was groundwater closed by level a three-way was measured valve by that a sighting was opened pipe. before the flood experiments.The lysimeters The groundwater were set in level the pit was using measured a crane. byDuring a sighting the lysimeter pipe. construction, when a layer Theof approximately lysimeters were 0.3 m set of soil in the was pit added, using the a crane.soil was During wetted theto saturation. lysimeter During construction, the construction, when a layer soil moisture probes and the neutron probe access tube were installed. When all the soil layers were of approximately 0.3 m of soil was added, the soil was wetted to saturation. During the construction, in place, up to the top of the soils, the excess water was allowed to drain to the bottom of the soil soil moisturecylinder for probes two days and and the neutronwas pumped probe out access with a tube vacuum were pump. installed. When all the soil layers were in place, upFive to the flood top depth of the treatments soils, the excess(1, 3, 5, water and 10 was cm) allowed and three to initial drain groundwater to the bottom level ofthe treatments soil cylinder for two(200, days 250, andand was300 cm) pumped were performed. out with aThe vacuum soil moisture pump. content was measured by a neutron probe Five(CNC503DR, flood depth Keyuan) treatments at 14 depths (1, 3, 5,(10, and 30, 10 50, cm) 70, and 90, three110, 130, initial 150, groundwater 170, 190, 210, level 230, treatments 250, (200,and 250, 270 and cm). 300 The cm) soil were surface performed. was 0 cm; Thethe soil soil column moisture bottom content was was370 cm. measured by a neutron probe (CNC503DR, Keyuan) at 14 depths (10, 30, 50, 70, 90, 110, 130, 150, 170, 190, 210, 230, 250, and 270 cm). 2.3. Soil Properties The soil surface was 0 cm; the soil column bottom was 370 cm. Silt loam soils (0–100 cm), which are the dominant soil type in the region, were collected from 2.3. Soilthe Propertiesupper reaches of Tarim River (Figure 1) and tested in the experiments. All soils samples were mixed and dried before placing into the lysimeters. The silt loam soil consisted of 41.53% sand and Silt loam soils (0–100 cm), which are the dominant soil type in the region, were collected from 53.62% silt, therefore the experiment represented most soils in the overflowing area of the Tarim the upper reaches of Tarim River (Figure1) and tested in the experiments. All soils samples were River. The soils contained 4.094 g kg−1 soil organic matter, 0.267 g kg−1 total nitrogen, and 0.604 g kg−1 mixedtotal and phosphorus. dried before The placing soils had into a the pH lysimeters. of 7.54, anThe electric silt loamconductivity soil consisted of 0.362 of ms 41.53% cm−1, sandand and 53.62%1.053 silt, g thereforekg−1 solute the salts. experiment The saturated represented soil water most content soils was in 46%, the overflowing and the bulk areadensity of thewas Tarim 1.54 gRiver. − − − The soilscm−1.contained The soil water 4.094 retention g kg 1 curvesoil organic was determined matter, 0.267 (six replications) g kg 1 total using nitrogen, a 15 Bar and Pressure 0.604 g kgPlate1 total phosphorus.Extractor The(1500F1, soils Soil had Moisture a pH of 7.54,Equipment an electric Corp, conductivitySanta Barbara, of CA, 0.362 USA), ms cmand− 1the, and results 1.053 are g kg−1 soluteshown salts. in The Figure saturated 4. Particle soil watersize distribution content was was 46%, determined and the using bulk densitya Malvern was Mastersizer 1.54 g cm −S 1laser. The soil waterdiffractometer retention curve (Malvern was determined Instrument, (sixMalvern, replications) UK) that using measures a 15 the Bar volume Pressure percent Plate of Extractor particles (1500F1, in Soil Moisture100 size classes Equipment from 0.02 Corp, to 2000 Santa μm. Barbara, CA, USA), and the results are shown in Figure4. Particle size distribution was determined using a Malvern Mastersizer S laser diffractometer (Malvern Instrument, Malvern, UK) that measures the volume percent of particles in 100 size classes from 0.02 to 2000 µm. Water 2017, 9, 523 6 of 15 Water 2017, 9, 523 6 of 15

FigureFigure 4. 4. SoilSoil water water retention retention curve curve and and particle particle size size di distributionstribution of of the the soil soil used used in in the the experiment. experiment.

2.4.2.4. Numerical Numerical Simulation Simulation SoilSoil water water movement movement in in the the experimental experimental field ﬁeld was was simulated simulated using using the the Richards Richards model, model, which which numericallynumerically solves solves the the governing governing flow ﬂow equation: equation:

∂θθ ∂  ∂ hh = K + 1 (1)(1) ∂tzt ∂z∂ z z 3 −3 wherewhere θθ isis the volumetric soil soil water content (cm 3 cmcm−3); );h his is the the pressure pressure head head (cm); (cm); and and kk(θ(θ) )is is the the −1 unsaturatedunsaturated hydraulic hydraulic conductivity conductivity function function (cm (cm day −1).). The The left-hand left-hand side side of of the the Richards Richards equation equation waswas expressed expressed in in terms terms of of moisture moisture content, content, rather rather than than in in terms terms of of the the hydraulic hydraulic head. head. For For this, this, we we referredreferred to to the the Hydrus-1D Hydrus-1D model model [52]. [52]. SoilSoil hydraulichydraulic propertiesproperties were were described described using us theing van the Genuchten–Mualem van Genuchten–Mualem analytical analytical functions functionsin Reference in Reference [53]: [53]: ( θs−θr θr + n m ; h < 0; θ(h) = [θθ1+|αh| ] (2) θ;0;θ ≥ srh  r s; h 0 m n θh  1α h m 2 0.5h 1/m i (2) K(θ) =KsSe 1 − 1 − Se (3) θ;s h  0 (θ − θr) Se = , m = 1 − 1/n, n > 1 (4) (θs − θr) m 2 3 0.5−3 1/m 3 −3 where θs is the saturated water contentKKSSθ11 (cmcm ); θr is the residual water content (cm cm ); K(3)s is  s ee the saturated hydraulic conductivity (cm day−1); and α and n are the shape parameters. The soil water dynamics in the experimental field were evaluated as a one-dimensional movement. θθ r The depth of the simulation profileSmnne was 3.2 m (soil,11/,1 column was 3.2 m in height), which was discretized(4) θθsr into a grid of 0.01 m. 3 −3 3 −3 where(1) Initial θs is conditionthe saturated water content (cm cm ); θr is the residual water content (cm cm ); Ks is the saturated hydraulic conductivity (cm day−1); and α and n are the shape parameters. TheThe changesoil water of the dynamics soil moisture in the content experimental with depth field under were the evaluated initial condition as a one-dimensional was defined as: movement. The depth of the simulation profile was 3.2 m (soil column was 3.2 m in height), which θ(z, 0) = θ (z) (5) was discretized into a grid of 0.01 m. 0

(1)where Initialθ0(z condition) is the initial measured soil water content. (2)The Upper change boundary of the conditionssoil moisture content with depth under the initial condition was defined as: The atmospheric boundary condition was assigned at the upper boundary [52]: θ,0θzz 0  (5) ∂θ ∂q where θ() is the initial measured soil water content.= − (6) ∂t ∂x (2) Upper boundary conditions where q is the surface water ﬂux at the time of ﬂooding. The atmospheric boundary condition was assigned at the upper boundary [52]:

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To simulate the level and flux of ponding water at the top of the lysimeter, the upper boundary was as follows:  h(0, t) = zxjl(i), 0 < zxjl(i)  zxjl(i) = zxjl(i − 1) − (RS(i) + E0) × dt(i), zxjl(i) < ZXNL (7)  zxjl(i) = ZXNL, ZXNL ≤ zxjl(i) where zxjl(i) and zxjl(i − 1) are the depths of ponding at i and (i − 1) (cm); ZXNL is the retention −1 capacity (mm); RS(i) is the infiltration intensity (cm min ); E0 is the water surface evaporation intensity (mm) observed by evaporation pan (E601); and dt(i) is the time step (min). Ponding may occur as ZXNL exceeded zxjl(i), which might be caused by the retention from runoff under excess infiltration capacity. The infiltration process is considered as pressured infiltration when there is surface water in the soils, and the hydraulic head equal to the ponding water depth [54], otherwise ponding produces surface runoff when the ponding water depth reaches the maximum value, which is equal to the retention capacity. (3) Bottom boundary conditions

The bottom boundary condition at z = Z0 was deﬁned as:

∂h = 1, z = Z (8) ∂z 0

(4) Evapotranspiration model and parameters If no ponding water occurs on the soil surface, the solution methods were referred to those used for the Hydrus-1D model [52], where the evaporation intensity E(t)[55] was calculated by Equation (9): ( E(h ) = −E exp(2 × 10−4(h − h )), h < h 0 0 0 e 0 e (9) E(h0) = −E0, h0 > he where h0 is the negative pressure at 10 cm below the soil surface (mm H2O); he is the critical negative pressure equivalent to the negative pressure of the ﬁeld capacity (with the negative pressure of the soils in this experiment being −2580 mm H2O at ﬁeld capacity); Ep is the potential evaporation intensity; and E0 is the water surface evaporation intensity (mm). The model parameters are shown in Table1. Parameters Ks and θs were measured, while parameters θr, α, and n were estimated by the soil water retention curve. The length interval of the simulated domain was 1 cm, and a variable time interval that was automatically adjusted in terms of the convergence of iterative process was used (0.001 d–1 d). The code was edited by Matlab 7.0 (Portola Valley, CA, USA) for Windows.

Table 1. Soil hydraulic parameters of the van Genuchten–Mualem model in the experiment.

Residual Soil Saturated Soil Saturated Hydraulic Parameter Exponent Water Content Water Content Conductivity −1 −1 θr (-) θs (-) α (cm ) n Ks (cm·day ) 0.16 0.48 0.0026 1.64 61.68

(5) Limiting assumption and uncertainty In operation, the model assumed that: (1) the soil properties were homogeneous; (2) soil was bare without a vegetation cover, and the plant root was sparse; (3) the study area was far away from the river so the increase in the groundwater table due to lateral water ﬂow was neglected; (4) the soil water movement was assumed to occur in honogeneous media so the impact of soil macropore ﬂow Water 2017, 9, 523 8 of 15

In operation, the model assumed that: (1) the soil properties were homogeneous; (2) soil was Water 2017bare, 9 ,without 523 a vegetation cover, and the plant root was sparse; (3) the study area was far away8 of 15 from the river so the increase in the groundwater table due to lateral water flow was neglected; (4) the soil water movement was assumed to occur in honogeneous media so the impact of soil was neglected;macropore and flow (5) was the neglected; influence ofand ponding (5) the waterinfluence on of soil ponding structure water and on porosity soil structure was neglected, and and changesporosity to was the neglected, parameters and of changes the VG to model the parame by pondingters of the water VG model pressure by ponding were not water considered. pressure were not considered. 3. Results and Discussion 3. Results and Discussion The experimental results showed that the flood water depth significantly contributed to The experimental results showed that the flood water depth significantly contributed to groundwater recharge and that various flood water depths had different effects under different groundwater recharge and that various flood water depths had different effects under different initialinitial groundwater groundwater tables tables (Figure (Figure5). Under5). Under flood flood water water depths depths ofof 1, 3, 3, 5 5 and and 10 10 cm, cm, the the groundwater groundwater table rosetable 15.1, rose 48.6,15.1, 76.948.6, and76.9 142.5and 142.5 cm, cm, respectively respectively when when the the initial initial groundwater groundwater table table waswas atat 200 cm; the groundwatercm; the groundwater table rose table 9.5, rose 36.5, 9.5, 69.5 36.5, and 69.5 108.1 and cm, 108.1 respectively cm, respectively when when the initial the initial water water table table was at 250 cm;was and at the250 groundwatercm; and the groundwater table rose 0.0, table 23.7, rose 64.6 0.0, and 23.7, 87.1 64.6 cm, and respectively, 87.1 cm, respectively, when the when initial the water table wasinitial at water 300 cm. table When was the at flood300 cm. water When depth the increased,flood water the depth height increased, of the groundwater the height of table the rise increased.groundwater It was found table thatrise increased. there was It a strongwas found relationship that there between was a strong the flood relationship water depth between and the height of theflood water water table depth rise, and and the height determination of the water coefficients table rise, and (R2 )the was determination 0.997 when thecoefficients initial groundwater (R2) was table was0.997 at when 200 cm, the 0.967initial when groundwater the initial table groundwater was at 200 cm, table 0.967 was when at 250 the cm, initial and groundwater 0.900 when the table initial was at 250 cm, and 0.900 when the initial groundwater table was at 250 cm. groundwater table was at 250 cm.

160

1cm 140 3cm 5cm 120 10cm

100

40 Height of water table rise (cm) (cm) rise table water of Height

0 200 250 300 Initial goundwater table(cm) Figure 5. The height of the groundwater table rise under different initial depths of groundwater table Figure 5. The height of the groundwater table rise under different initial depths of groundwater table and amount of flood. and amount of flood. Similar results were reported by Sorman and Abdulrazzak [24], who suggested that flood Similarrunoff resultsvolume wereand reportedduration bywere Sorman the dominant and Abdulrazzak factors influencing [24], who the suggested cumulative that infiltrated flood runoff volumevolume and durationand recharge were of theshallow dominant groundwater factors tables influencing. Morin et the al.cumulative [1] showed the infiltrated importance volume of the and rechargerole of of shallow large and groundwater medium floods tables. in aquifer Morin etrechar al. [1ge;] showed the relative the importancecontribution ofof thehigh-magnitude role of large and floods to groundwater recharge also increased, whereas medium and small floods contributed medium floods in aquifer recharge; the relative contribution of high-magnitude floods to groundwater relatively less. Lange [56] reported that high-magnitude floods were mostly significant for rechargegroundwater also increased, recharge whereas under the medium Kuiseb andRiver. small The above-mentioned floods contributed research relatively was similar less. Lange to our [56] reportedresearch that results. high-magnitude floods were mostly significant for groundwater recharge under the Kuiseb River.The Theslope above-mentioned of the relationship researchbetween wasthe flood similar water to ourponding research depth results. and height of the water Thetable slope rise ofat thethe relationshipinitial groundwater between tables the flood of 200 water, 250, ponding and 300 depthcm were and 13.99, height 10.85, of the and water 9.65, table rise atrespectively. the initial groundwater These results tablesindicated of 200,that 250,the height and 300 of cmthe werewater 13.99,table rise 10.85, was and closely 9.65, related respectively. to Theseflood results water indicated depth; thatif the the initial height water of thetable water was d tableeeper, rise the was variation closely in relatedthe depth to floodof the water table depth; if the initial water table was deeper, the variation in the depth of the water table rise was smaller. Sorman and Abdulrazzak [24] also reported that the relationship between the ground water recharge and flood inflow volume could be represented by linear regression equations, with the correlation coefficients of 0.76, but the slope was 0.302, which was still lower than the slope related to flood recharge infiltration in our research. Note that Sorman and Abdulrazzak [24] experiments were conducted under conditions of possible surface water runoff, while our experiments were conducted in lysimeters with ponding water, with no occurrence of outflow. Water 2017, 9, 523 9 of 15

rise was smaller. Sorman and Abdulrazzak [24] also reported that the relationship between the ground water recharge and flood inflow volume could be represented by linear regression equations, with the correlation coefficients of 0.76, but the slope was 0.302, which was still lower than the slope related to flood recharge infiltration in our research. Note that Sorman and Abdulrazzak [24] experiments were conducted under conditions of possible surface water runoff, Waterwhile2017, 9our, 523 experiments were conducted in lysimeters with ponding water, with no occurrence9 ofof 15 outflow. A recharge process following a flood event is usually comprised of three processes stages: the risingA recharge process processof the groundwater following a floodtable; the event peak is usuallypart of the comprised groundwater of three table; processes and the stages:declining the risingpart process or recession of the groundwaterprocess [57]. Figure table; the6 clearly peak partdisplays of the the groundwater temporal changes table; andprocess the decliningof the water part or recessiontable; the processgroundwater [57]. Figure table rises6 clearly rapidly displays after fl theooding, temporal reaching changes maximum process values of the after water 4.0–10.5 table; theh groundwaterwith a mean table value rises of rapidly 6.16 h (SE after of flooding, 0.82 h) for reaching all tests, maximum indicating values that the after groundwater 4.0–10.5 h withresponded a mean valuequickly of 6.16 to hfloods (SE of in 0.82 these h) forexperiments, all tests, indicating which were that consistent the groundwater with results responded from Doble quickly et al. to floods[26]. in theseDoble experiments, et al. [26] also which reported were that consistent overbank with flood results recharge from was Doble distinct et al. from [26]. hydrostatic Doble et al. loading [26] also reported(the pressure that overbank at any flooddepth recharge due to hydrostatic was distinct pressures), from hydrostatic which is loading characterized (the pressure by a rapid at any rise depth in duegroundwater. to hydrostatic Dahan pressures), et al. [5] which observed is characterized that a flood by caused a rapid the rise water in groundwater. level to rise after Dahan 5.5 eth al.and [5 ] observedreached that peak a floodwater causedlevel in the hyper-arid water level area to riseof the after Arava 5.5 Valley, h and reachedwhich is peakalso in water agreement level with in the hyper-aridour research. area of the Arava Valley, which is also in agreement with our research.

0 0 Water table at 200 cm Water table at 250 cm 1 cm 1 cm 3 cm 3 cm 5 cm 100 5 cm 100 10 cm 10 cm

200 200 Groundwater table Groundwater (cm) Groundwater table (cm) table Groundwater

300 300

012345 012345 Days after flooding (d) Days after flooding (d)

0 Water table at 300 cm 1 cm 3 cm 5 cm 10 cm 100

200 Groundwater table (cm) table Groundwater

300

012345 Days after flooding (d)

Figure 6. The response of the groundwater table to flood depth over time. Figure 6. The response of the groundwater table to ﬂood depth over time.

As soon as the flood ceased, the infiltration process stopped, the vadose zone was partially drained,As soon and as the the groundwater flood ceased, level the started infiltration to decrease process toward stopped, a new the water vadose level zone that was was partiallyhigher drained,than the and initial the groundwater one [58]. When level the started groundwater to decrease table towardreached a a new maximum water levelvalue that after was a flood higher water than therecharge, initial one the [58 ].water When table the groundwaterstarted falling table due reached to evaporation, a maximum and value the water after afalling flood watervelocity recharge, was thesignificantly water table startedsmaller fallingthan the due water to evaporation, table rise andvelocity. the waterIn our falling experiment, velocity even was significantlywhen the smallergroundwater than the watertable was table at rise a depth velocity. of 3.2 In m our (the experiment, highest groundwater even when thetable), groundwater evaporation table was was still at a depth of 3.2 m (the highest groundwater table), evaporation was still present. This was the reason why the evaporation rate was much lower than the ponding water infiltration rate. The soil used in our experiment was a sandy loam with a high infiltration rate (saturated hydraulic conductivity was 61.68 cm d−1); however, the measured water surface evaporation rate ranged from 3.1 to 15.7 mm d−1, with a mean value of 6.3 mm d−1 during the experiment. Xu et al. [59] also reported that groundwater rise velocity was larger than the water falling velocity after flooding in the lower reaches of the Tarim River, and the water rise velocity was slower for deeper water levels, which was similar to our research results. Water 2017, 9, 523 10 of 15

If the initial groundwater table was deeper, the range of change in the groundwater table was smaller (Figure5), which indicated that the flood ponding water had less impact on deeper groundwater tables. Wu et al. [57] discovered that when the groundwater table was shallow, flood water percolated into the groundwater relatively quickly, which was consistent with our results. For example, in our experiment, the elapsed time of the water table reaching peak water level was 3.6 h at an initial groundwater table depth of 200 cm; however, it was 10.1 h at an initial groundwater table depth of 300 cm. When the groundwater table was very deep, the variations in the recharge rate became imperceptible [57]. Our results partly supported this conclusion; however, we observed that a relatively small amount of flood water still slightly recharged a deep groundwater table. For example, the deepest groundwater table was 3.0 m in our research, and if the flood water depth was 1 cm, a rise in the groundwater table was not observed; however, if the flood ponding water was 10 cm, a rise in height of 87.07 cm was measured. Though groundwater tables deeper than 3.0 m were not examined in our experiment, Xu et al. [59] reported that the groundwater table increased from 9.87 to 3.16 m after a flood in the Tarim River. If the initial groundwater table was deeper and the water table drop was slower, it may have been due to less evaporation of the deeper water table. Warrick [60], Gardner and Fireman [61] and Thorburn et al. [62] found that the relationship between the depth of the groundwater table and the amount of evaporation from soils was an inverse power relationship, indicating that deeper groundwater tables had smaller water consumptions due to evaporation. This provided a good explanation as to why the deeper water tables had slower falling velocities. The water table fall was more rapid at the initial stage and then dropped slowly (Figure5). This was explained by the fact that the water table was initially higher due to flood water recharge; in turn, the higher groundwater table experienced more evaporation, which contributed to the rapid falling; as a result, the groundwater table dropped to a deeper level, evaporation was reduced, and the groundwater falling velocity was slowed. Dahan et al. [58] reported that the rising rates of the groundwater level were 1.38 and 1.4 cm h−1 for two floods in the Kuiseb River. Our results showed higher rising rates, which may be due to the fact that our experiments were conducted in lysimeters containing homogeneous soil and water movement was a one-dimensional process with homogeneous soil. Dahan et al. [58] conducted their experiments under larger-scale field conditions, which were likely affected by lateral flow within a layered profile of alluvial deposits with possible preferential flow. Numerical simulations based on a system of Equations (1)–(9) showed good agreement between the simulated and measured groundwater levels with an R2 and an RMSE of 0.87 and 63.91 cm, respectively, which suggested a high consistency between the simulated and experimentally observed processes (Figure7). Similar to our results, Rennolls et al. [63,64] and Barnes et al. [65] employed regression models to estimate the infiltration recharge of groundwater at relatively shallow water tables. The rise in the height of the groundwater table—as affected by flood water depths under six initial groundwater tables—and four initial soil water contents were simulated, and the results are shown in Figure8. By considering the influence of various hydrological characteristics, regression equations have been suggested to estimate the groundwater recharge [24]. Therefore, a regression equation between the initial groundwater table, initial soil water content, flood water depth and water table rise was fitted using the results of the numerical model:

H = −10.639H0 + 23.552w0 + 23.989h − 90.439, (H > 0) (10) where H is the groundwater height rise; H0 is the initial groundwater depth (cm); w0 is the initial soil water content (cm); and h is the ﬂood water depth (cm). Water 2017, 9, 523 11 of 15

Water 2017, 9, 523 11 of 15 Water 2017, 9, 523 11 of 15

Figure 7. Observed vs. simulated ground water table and 1:1 line.

Similar to our results, Rennolls et al. [63,64] and Barnes et al. [65] employed regression models to estimate the infiltration recharge of groundwater at relatively shallow water tables. The rise in the height of the groundwater table—as affected by flood water depths under six initial groundwater tables—and four initial soil water contents were simulated, and the results are shown in Figure 8. By considering the influence of various hydrological characteristics, regression equations have been suggested to estimate the groundwater recharge [24]. Therefore, a regression equation between the initial groundwater table, initial soil water content, flood water depth and water table rise was fitted using the results of the numerical model:

= −10.639 + 23.552 + 23.989ℎ − 90.439, ( > 0) (10) where is the groundwater height rise; is the initial groundwater depth (cm); is the initial soil water content (cm);Figure andFigure 7. ℎObserved is7. Observedthe flood vs. vs. simulated water simulated depth ground ground (cm). water water tabletable and 1:1 1:1 line. line.

Similar to our results, Rennolls et al. [63,64] and Barnes et al. [65] employed regression models Initial groundwater depth at 200cm Initial ground water depth at 250cm to estimate the infiltration recharge of groundwater at relatively shallow water tables. The rise in the height of the groundwater table—as affected by flood water depths under six initial groundwater

tables—and four initial soil water contents were simulated, and the results are shown in Figure 8. By

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a m 200 116 m 94 w

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200 u 114 e u 92 te t

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c G G 90 r 112 r 250 e 300 te 89 t 111 a a 12 w 10 w 10 l 8 88 il 110 i o 8 o where is the groundwater6 height rise;s is the initial groundwater6 depth109 (cm); s is the initial 87 l l F 4 Floo 4 ia lood ia d p tt po 2 86 it ond 2 108 i ndin n ing 0 In g he ℎ 0 I heigh soil water content (cm); andights is the flood water depth (cm). ts (c (cm) m)

Initial ground water depth at 300 cm Initial ground water depth at 400 cm

Initial groundwater depth at 200cm Initial ground water depth at 250cm

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Initial ground water depth at 300 cm Initial ground water depth at 450 cm InitialInitial ground ground water depth depth at at 400 500 cm cm

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Figure 8. NumericalNumerical simulation simulation results results of of the the groundwater table table rise height as affected by ﬂoodflood ponding heights under different initialinitial groundwatergroundwater tabletable depthdepth andand soilsoil waterwater content.content.

The R2 and RMSE values were 0.789 and 80.80 cm, respectively. Sorman and Abdulrazzak [24] reported that larger contributions may occur as a result of high initial soil moisture content accumulated from sequences of flood events and a shallow groundwater table. Our regression equation showed that was positively correlated to ℎ and , but was negatively correlated to ; therefore, our results fully agreed with this conclusion. The frequency of overbank flooding also determines the significance of this recharge to the water balance [26]. When planning schemes of groundwater recharge, it is especially important to know the maximum volume of water that can recharge the groundwater in a given period of time [7]. This is of great concern in planned or regulated flooding (e.g., ecological gates) for determining the time of artificial watering and the amount of artificial flooding in the Tarim River. Therefore, the artificial watering time interval was also considered in our study. The relationship between the flood time interval, initial groundwater table, initial soil water content, flood water amount and depth of the water table rise was fitted using numerical model results:

= −0.168 + 2.532 + 1.415ℎ − 0.990 − 14.131, ( > 0) (11) where is the time interval (day). The R2 and RMSE values were 0.892 and 115.964 cm, respectively. Our results were obtained on a silt loam with a high infiltration rate; Li et al. [35] reported that the infiltration rate was high in the entire floodplain of the Tarim River. Glenn et al. [66] found soils that contained 49.23% sand and 14.23% clay in a US riparian zone; Ghazavi et al. [25] observed soils composed of 62.05% sand and 8.17% clay in a floodplain in Iran; and Dahan et al. [58] reported soils consisting of 90.82% sand and 1.80% clay at a depth of 0–6 m in Israel. In general, soils with low clay and high sand proportions showed high infiltration rates [67]. All these reports suggested that soils with high infiltration rates were common in hyper-arid river floodplains. Our model demonstrated a good performance for sandy loam in the Tarim River; however, as this model was applied to other soil types and regions, it required local input parameters (θs, θr, Ks, α, n) of the VG model. If soil texture was different across the various layers, those parameters were measured in the different layers, and all input parameters were associated with given soil layers. A fixed space interval was recommended (e.g., 1–3 cm). In contrast, a variable time interval that could be automatically adjusted in terms of the convergence of the iterative process was suggested. Therefore, our research results could serve as a reference for other floodplains in hyper-arid regions across the world.

4. Conclusions We studied flood water infiltration and groundwater recharge using lysimeters and numerical models in the floodplains of the Tarim River, China. The different flood water depths resulted in different heights of the groundwater table rise under different initial groundwaters. It was found that there was a strong relationship between the flood water ponding depth and the height of the water table rise. The groundwater responded quickly to flood water, and its falling velocity was

Water 2017, 9, 523 12 of 15

The R2 and RMSE values were 0.789 and 80.80 cm, respectively. Sorman and Abdulrazzak [24] reported that larger contributions may occur as a result of high initial soil moisture content accumulated from sequences of flood events and a shallow groundwater table. Our regression equation showed that H was positively correlated to h and w0, but was negatively correlated to H0; therefore, our results fully agreed with this conclusion. The frequency of overbank flooding also determines the significance of this recharge to the water balance [26]. When planning schemes of groundwater recharge, it is especially important to know the maximum volume of water that can recharge the groundwater in a given period of time [7]. This is of great concern in planned or regulated flooding (e.g., ecological gates) for determining the time of artificial watering and the amount of artificial flooding in the Tarim River. Therefore, the artificial watering time interval was also considered in our study. The relationship between the flood time interval, initial groundwater table, initial soil water content, flood water amount and depth of the water table rise was fitted using numerical model results:

H = −0.168H0 + 2.532w0 + 1.415h − 0.990dt − 14.131, (H > 0) (11) where dt is the time interval (day). The R2 and RMSE values were 0.892 and 115.964 cm, respectively. Our results were obtained on a silt loam with a high infiltration rate; Li et al. [35] reported that the infiltration rate was high in the entire floodplain of the Tarim River. Glenn et al. [66] found soils that contained 49.23% sand and 14.23% clay in a US riparian zone; Ghazavi et al. [25] observed soils composed of 62.05% sand and 8.17% clay in a floodplain in Iran; and Dahan et al. [58] reported soils consisting of 90.82% sand and 1.80% clay at a depth of 0–6 m in Israel. In general, soils with low clay and high sand proportions showed high infiltration rates [67]. All these reports suggested that soils with high infiltration rates were common in hyper-arid river floodplains. Our model demonstrated a good performance for sandy loam in the Tarim River; however, as this model was applied to other soil types and regions, it required local input parameters (θs, θr, Ks, α, n) of the VG model. If soil texture was different across the various layers, those parameters were measured in the different layers, and all input parameters were associated with given soil layers. A fixed space interval was recommended (e.g., 1–3 cm). In contrast, a variable time interval that could be automatically adjusted in terms of the convergence of the iterative process was suggested. Therefore, our research results could serve as a reference for other floodplains in hyper-arid regions across the world.

4. Conclusions We studied flood water infiltration and groundwater recharge using lysimeters and numerical models in the floodplains of the Tarim River, China. The different flood water depths resulted in different heights of the groundwater table rise under different initial groundwaters. It was found that there was a strong relationship between the flood water ponding depth and the height of the water table rise. The groundwater responded quickly to flood water, and its falling velocity was significantly less than its water rise velocity. If the initial groundwater table was deeper, the range of change in the groundwater table was smaller.

Acknowledgments: This research was supported by the National Natural Science Foundation of China (Grant No. 41571035) and the Youth Innovation Promotion Association CAS (2017477). Author Contributions: “Xinhu Li and Gary Feng conceived and designed the experiments; Xinhu Li performed the experiments; Xiaoyu Pi and Congbao Xie analyzed the data; Xinhu Li contributed materials; Guohua Zhang wrote the paper.” Authorship must be limited to those who have contributed substantially to the work reported. Conﬂicts of Interest: The authors declare no conﬂict of interest. Water 2017, 9, 523 13 of 15

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