Mimic pulse-base flows and groundwater in a regulated river in semiarid land: Riparian restoration issues

Ecological Engineering 83 (2015) 239–248

Contents lists available at ScienceDirect

Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng

Mimic pulse-base flows and groundwater in a regulated river in semiarid land: Riparian restoration issues Jorge Ramírez-Hernándeza,* , Jesús E. Rodríguez-Burgueñoa , Francisco Zamora-Arroyob , Concepción Carreón-Diazcontia , Dennice Pérez-Gonzálezc a b c

Instituto de Ingeniería, Universidad Autónoma de Baja California, Calle de la Normal s/n Col. Insurgentes Este, Mexicali, Baja California 21280, Mexico Sonoran Institute, 44 E. Broadway Blvd., Suite 350, Tucson, AZ 85701, United States Facultad de Ingeniería Mexicali, Universidad Autónoma de Baja California, Mexico

A R T I C L E I N F O

A B S T R A C T

Article history: Received 19 November 2013 Received in revised form 10 May 2015 Accepted 8 June 2015 Available online 4 July 2015

River regulation in semiarid lands often results in a dry riverbed, which conducts to serious ecological degradation. Given the numerous hydraulic control structures, in the U.S. area, and the water diversions for consumptive uses, in Mexican territory, nowadays the Colorado River Delta region shows runoff only during wetting years when surplus water is available. Due to concerns related to the ecological health of this Delta, an international agreement (Minute 319) was signed between Mexico and the U.S. in November 2012. This agreement contemplates, among other issues, the release of water for environmental restoration purposes in the Colorado River Delta riparian corridor. The purpose of this study is to propose feasible discharge flows, timing, and extension of flooded areas for riparian restoration. This is accomplished through the analysis of sediments texture, soil salinity, and groundwater levels, which were conducted in conjunction with diverse base and pulse flow modeling scenarios in a 10-mile reach of the Colorado River. Infiltration from irrigation channels, irrigation returns, and river discharge flows was recognized and the depth to water table and its influence on riparian vegetation was analyzed. Under a pulse flow of 200 m3 s 1, the resulting flood will cover 65% of the entire study area, similarly to the inundation observed in 1997, the year of the last natural flood. Modeling results further suggest that, the mainstream of the river will be flooded by mimic base flows of 10 m3 s 1. In addition, the timing and elapsed time of discharges that mimic base and pulse flows could play an important role to maintain shallow groundwater levels and to promote native riparian vegetation establishment. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Colorado River Delta Groundwater–surface water interactions Pulse-base flows Regulated rivers Riparian restoration

1. Introduction Hydrologic regimes strongly influence the structure and function of riparian vegetation (Poff et al., 1997), for instance, flow-regulated riparian zones have been damaged by the loss of both, perennial and flood pulse regimes. Such is the case of the south-western United States where the flood timing for seedling establishment, the threshold flood for recruitment, the groundwater depth tolerance of some riparian species, and the seedling and growth response to water quality have been impacted by rivers fragmentation and flow regulation (Stromberg, 1998). The Colorado River is perhaps the most regulated river in the world. Its water rights have been carved up to allocate 18,502 hm3 of water to seven U.S. states and another 1850 hm3 to Mexico, for a

* Corresponding author. Fax: +52 686 5664150. E-mail address: [email protected] (J. Ramírez-Hernández). http://dx.doi.org/10.1016/j.ecoleng.2015.06.006 0925-8574/ ã 2015 Elsevier B.V. All rights reserved.

total of 20,352 hm3 per year (Getches, 2003). However, tree rings studies covering a 400-year period indicate that the average flow of water is actually about 16,652 hm3 per year. Therefore, the current allocations greatly exceeds the river’s average flow (Stockton and Jacoby, 1976). Water volumes allocated to Mexico are completely diverted for consumptive uses. Nevertheless, given the rare case of a very wet year when surplus water is not captured by any of its users (extreme flow events), it may flow freely along the river channel and even reach the ocean. Nowadays, the Colorado River and its riparian corridor, from Morelos dam to the Gulf of California (approx. 145 km), as any other regulated rivers in semiarid lands, are completely dry in most of the reaches. However, the river channel in the upper part of limitrophe reach (Morelos dam to South International Boundary, SIB) has perennial water due to groundwater seepage (Ramírez-Hernández et al., 2013). Downstream Morelos dam, from kilometer 15 to 75, the entire riparian area is becoming dry

240

J. Ramírez-Hernández et al. / Ecological Engineering 83 (2015) 239–248

given that the groundwater level deepens and disconnects from the riverbed. The Channel reach between 75 and 92 km contains perennial water derived from irrigation returns. The most important restoration site and our study area are in this reach. From this point to the Hardy River confluence, the channel become dry again. Records from 1948 to 2009 at Morelos Dam (Fig. 1) showed maximum temperatures reaching over 50  C, an annual average precipitation of just 52 mm, and evaporation rates between 3.2 and 1.9 m annually, (CONAGUA, 2012). For more than fifty years, a fraction of the Colorado River water within the U.S. has been dedicated to environmental affairs. Nevertheless, it was just during the past decade that a new perception on shared environmental issues gave way to discussions on water assignations for joint environmental efforts in Mexican land. Furthermore, agreements on water management for environmental concerns in Mexico have been lately included in several binational water negotiations (IBWC, 2000, 2010a,b,c, 2012). The most recent agreement, signed in November 2012,

involved representation of both, the United States and México, and participation of several non-governmental organizations. It contemplates implementing a five-year pilot program designed to mimic the Colorado River’s base and pulse flows by releasing 195 hm3 of water into the Mexican dry portion of the river course itself (IBWC, 2012). Consequently, with the general purpose of assisting the ongoing conservation–restoration efforts, we modeled discharge flows to mimic pulse and base flow scenarios in a reach of the Colorado River. The ultimate objective of this work is to determine the potential extension of flooded areas and the seasonal variation of groundwater depth. The river reach used for this study is our demonstration site, located between the Mexicali–Benjamin Gil railroad river bridge (Railroad Bridge, RB) and the Carranza town’s river crossing (Carranza Crossing, CC) (Fig. 1), which has become the most important restoration area on the Colorado River Delta. In addition, the kind of conservation–restoration methodology used at this site can be extrapolated to the entire Colorado River Delta and to any other rivers in semiarid regions.

Fig. 1. Colorado River Delta and study area.

J. Ramírez-Hernández et al. / Ecological Engineering 83 (2015) 239–248

2. Regional settings 2.1. The delta and riparian zones The Colorado River Delta area (CRD) comprises the lower portion of a deep alluvial basin (Fig. 1), which used to extend along 780,000 ha (Sykes, 1937). However, current environmental conditions are able to support only about 60,000 ha of wetlands (Luecke et al., 1999). Prior 1963, when several control structures (dams) were built, the Colorado River was a warm, sediment-heavy stream with tremendous seasonal fluctuation. Afterward, the river became much clearer and colder, with little fluctuation in annual flows, which highly altered the natural channel dynamics (Ohmar et al., 1977). It is known that the combined environmental impacts of river impoundment and diversion, ground-water pumping, spread of non-native vegetal species, farming, and other human activities have had a devastating effect on the river’s ecology (Briggs and Cornelius, 1998). Historically, the Colorado riparian corridor was dominated by a mesophytic native forest that included a high population of Populus fremontii (Fremont’s cottonwood), Prosopis spp., and Salix gooddingii (Goodding’s willow) trees and numerous underbrush (Busch and Smith, 1995; Ohmart et al., 1988). Nowadays, riparianzone disturbances such as the lack of surface flows over long periods of time and a new hydrogeological dynamic, unfavorable for the riparian ecosystem requirements, have caused much of the flood plain terraces to be colonized by invasive, salt tolerant vegetation, such as Tamarix ramosissima and Pluchea sericea (Stromberg, 2001). Visual cover classification of the study area carried out by aerial photos from 2002 showed a coverage of more than 55% of T. ramosissima and P. sericea and only 1.1% of P. fremontii and S. gooddingii (Cupul-Magaña, 2009). 2.2. Historic and current flows in Colorado River Delta Before the establishment of riverbank communities (prior 1896), whose inhabitants initiated control and management of the Colorado River, the entire water flow reached the delta zone. Moreover, the flow system was highly dependent on watershed rainfall and snowmelt, which created a dynamic and unstable delta (Sykes, 1937). At that time, the river flow varied from near 0 to

241

6000 m3 s 1 with an annual average of 20.7  109 m3 calculated with data from 1896 to 1921 (Fradkin, 1981). During a subsequent drier decade (1931–1940), the average flow was 14.5  109 m3 year 1 (Fradkin, 1981). Extraordinary flows were also measured during the early 1980’s with a peak of approximately 800 m3 s 1 (Glenn et al., 2008). Flow recurrence intervals at the Mexican North International Boundary (NIB) have been estimated from 538 to 680 m3 s 1 for a 10-year flood; meanwhile, the 50-year flood varies from about 878 to 1350 m3 s 1 and the 100-year flood fluctuates between 878 and 1900 m3 s 1 (Glenn et al., 2008; Tetra Tech, 2004). In addition, it has been reported that both, the channel and the floodplains between levees along the limitrophe river section, from the NIB and the SIB (Fig. 1), have a capacity for a flood flow up to 3964 m3 s 1 (Tetra Tech, 2004). On the other hand, a hydraulic analysis at the RB–CC section indicates a floodplain maximum discharge capacity of 800 m3 s 1 (Quesada, 2007), as previously reported by the Mexican National Water Agency (CONAGUA, 2007). All known historical CRD flow data measured and/or estimated by several authors using different methods have been compiled for the first time ever (Table 1) and will provide the criteria for selection of modeling pulse flows. The minimum water volume useful for riparian vegetation in the CRD is 3200 hm3 with flow rates of 100–200 m3 s 1 (Luecke et al., 1999). This amount was assessed for the first time ever using data from January to April 1997 excess flow event. 2.3. Sediment in the Delta Prior to human impacts, the river supplied an average of over 150  106 tons of sediment per year (Meckel, 1975; Sykes, 1937). Under the current controlled (dry) conditions of the Colorado River, no sediments reach the CRD region. However, it has been established that the meandering channel-fill deposits in the CRD are of three kinds: active channel (sands fine to very fine with deposits of clay pebble clast), partial abandoned (well-sorted very fine sands and ripple-bedded silts with interlayered clays), and abandoned (laminated mud and silt) (Van Der Kamp, 1973). In addition, most farmland soils in the study area are affected by salinization. Salinity, measured as electric conductivity, for the

Table 1 Historic discharge main events and discharge proposed to base and pulse flows in the Colorado River Delta. Source Sykes, (1937) Fradkin, (1981) in Glenn et al. (1996) Fradkin, (1981) in Glenn et al. (1996) Glenn et al. (2008)

Discharge (m3 s 3

1

1

)

Zamora-Arroyo et al. (2001) CONAGUA (2012)

6000 (m s ) 20.7  109 m3 year 1 (656 m3 s 1) 14.5  109 m3 year 1 (459 m3 s 1) 800 (m3 s 1) 583–617 910–1370 1036–1954 583–617 (680-N) (m3 s 1) 880–1350 (878-N) (m3 s 1) 1000–1900 (m3 s 1) 140,000 cfs 100–200 (m3 s 1) 3200 hm3 80–120 (m3 s 1) 3000 hm3 800 (m3 s 1)

Quesada (2007)

800 (m3 s

Tetra Tech (2004)

Luecke et al. (1999)

1

)

Time period

Description

Historically 1896–1921

Maximum

1931–1940

Dried decade

1980’s 10 years 50 years 100 years 10 years 50 years 100 years Capacity (limitrophe reach)

Riparian reach. Peak flow Return period from modeling

January–April 1997

Useful to riparian vegetation in the Colorado River Delta. Observation

February–April 1997

Sufficient to inundate most of the floodplain, they documented water reaching the Gulf of California and Laguna Salada. Observation Peak flow, personal communication Eng. Trejo. Chief Engineer, CONAGUA. Measured at railroad bridge This study area reach, HEC-RAS modeling. n-values calibrated with field data measurements

1980’s Maximum discharge flow capacity

Riparian reach. Return period from modeling (Technical report)

242

J. Ramírez-Hernández et al. / Ecological Engineering 83 (2015) 239–248

upper 30 cm of soil varies from 16 dS m 1, in the northern portion, to <2 dS m 1 in the central part of the delta zone (Carrillo-Guerrero, 2009). Soil salinity in the southern portion ranges from 2 to 8 dS m 1. 2.4. Surface–groundwater interaction Before hydrologic modifications took place, the Colorado River supplied water to the local aquifer via regular direct infiltrations along the river channel bed and through riverbanks during annual flooding events (Dickinson et al., 2006). In fact, there are two distinct aquifers within the subsurface of the Mexicali valley. The upper groundwater system consists on a discontinuous sequence of fine and medium-grained alluvial sediments deposited by the Colorado River. It is generally non-confined, dependent on annual Colorado River seepage, extends from surface level to more than 2000 m deep, and covers the entire delta area (Ariel Construcciones, 1968; Díaz-Cabrera, 2001; Olmsted et al., 1973). The second aquifer is deeper, related to geothermal reservoirs, and is separated from the shallower water body by discontinues post-depositional geothermally altered shale layers (Elders et al., 1979; Lippmann et al., 1991). 2.5. Mimic base and pulse flows Natural base and pulse flows should no longer be expected for intensely regulated streams, such as the CRD area. Therefore, the estimation of discharge rates that may reproduce beneficial base and pulse flows has been a challenge for riparian restoration programs. Nevertheless, it is known that flood timing, flood stage, and the shape of the hydrograph are critical in determining the potential success of any riparian restoration effort (Hughes and Rood, 2003).

In general, base flow is the portion of streamflow that comes from seepage of groundwater into a channel, represents the source of running water in a stream during dry weather, and maintains longitudinal (along the stream) and vertical (with the aquifer) connections. Whereas dams and diversions affect rivers, impacts are particularly evident in lowland watercourses therefore, base flows are not expected to be continuous along the entire river length. In the case of dry river channels, when the aquifer no longer supplies water, such as those in arid lands, riparian vegetation is groundwater dependent and the role of the base flow is inverted, i.e., now it replenishes the groundwater table. To preserve vertical connectivity (surface–groundwater interactions) in dry reaches along the river, where the water table is deeper than the riverbed, it is necessary to maintain the base flow along the entire year, i.e., dry reaches of CRD riparian corridor. Hence, engineering a “mimic base flow” or “maintenance flow” (Hughes and Rood, 2003) must take into consideration both, the current source and timing of aquifer recharge (irrigation returns in this case) and the timing to promote riparian vegetation growth. Conversely, due to lateral connection between river and floodplain, the flood pulse is the major force controlling biota in the floodplain itself (Junk et al., 1989). High flows provide further ecological benefits by maintaining ecosystem productivity and diversity, removing and transporting fine sediments, importing woody debris into the channel, and connecting the channel to the floodplain. In addition, the scouring of floodplain soils rejuvenates habitat for plant species that germinate only on bare, wetter surfaces that are free of competition. Even more, large floods that occur on the order of decades inundate the aggraded floodplain terraces where lateral successional species establish (Poff et al., 1997). Flood flow on CRD depends on surplus water in the hydraulic system as evidenced by the last two extreme events during the ‘80s

Fig. 2. Location of piezometers of UABC and CILA, CILA and Laguna Grande restoration sites, vegetation profiles (VP-1 to 4), and polygon of study area.

J. Ramírez-Hernández et al. / Ecological Engineering 83 (2015) 239–248

and ‘90s (Table 1). Nevertheless, in 2001 the U.S. Colorado River Lower Basin States implemented guidelines for sharing surplus water instead of discharging it to the CRD. Therefore, there is a very low probability of a natural occurrence of flood pulses in the future hence, the riparian ecosystem entirely depends on engineered floods or “mimic pulse flows”. 3. Methodology 3.1. Topographic survey A topographic survey of the study area was conducted during 2010–2011 using a Topcon GPS double frequency model GR-3. It included all the flooded areas between levees, i.e., the riparian areas and bared soils, the floodplains, the flow channel, backwaters channels, and lagoons. More than 40,000 points were measured over a 6-month period. Topographic coverage outside the flow channel was supplemented by ground measurements and an airborne LIDAR survey. The last one was carried out by the National

243

Institute on Statistics, Geography and Informatics (INEGI by its initials in Spanish) after the April 2010 El Mayor–Cucapah earthquake. A digital elevation model (DEM) was obtained applying an ordinary Kriging interpolation with a spherical semivariogram model and a variable search radio using Geostatistical Analyst from ArcMap 9.2 (ESRI, 2006). In turn, this DEM was used to generate the geometric data required for HEC-RAS flow modeling and to describe the modern riparian terraces. Topographic data were also applied, along with information from a vegetation recognition campaign, to prepare a geomorphological description of the river floodplains along four selected transects (Fig. 2). 3.2. Soil sampling and classification Soil sampling was performed using a California auger type sampler and three different depths: 0.20 m, 1.0 m and water table depth. Location of sampling sites was based on randomly selected squares (100  100 m) using a Garmin hand-GPS, model eTrex1.

Fig. 3. Terraces and their relation with main channel and vegetation. CR: Colorado River main stream. HT: high terraces. MT: medium terraces. LT: low terraces. Elevation is in meter above sea level. See Fig. 2 for location of profiles. Vegetation density and water level in the river channel presented in the profiles was measured in the field.

244

J. Ramírez-Hernández et al. / Ecological Engineering 83 (2015) 239–248

Grain size distribution was obtained using a technique proposed by Bouyucos (1936), in accordance with the American Society for Testing and Materials Method D422-63 (ASTM-D422, 1963). Conductivity and pH analyses were conducted for all samples. Textural tri-linear diagrams of sand–silt–clays and Thiessen polygon maps of soil contents for each of the three depths were plot. Two consecutive soil sampling campaigns were implemented. The first one showed that 144 is the statistically representative sample size for a confidence interval of 95%; the second campaign was carried out to complete the required sample size. 3.3. Groundwater fluctuations Groundwater fluctuations were recorded monthly along eight transects with four piezometers respectively (Fig. 2), two on each side of the main river channel (piezom UABC). Another seven piezometers scattered along the CILA restoration site (piezom CILA) were also included in the monitoring network. Piezometers were installed using an auger type driller Little Beaver model H-10220 without perforation fluids to prevent soil permeability changes. The length of these piezometers was set 3 m below the groundwater level and each one was equipped with 4 inches PVC pipe screened along the last 2 m. The entire pipe was protected with polyester filter to reduce entry and accumulation of fine sediments. Monthly groundwater elevations have been obtained since 2006 though some intervals are incomplete due to vandalism on several piezometers. Potentiometric maps from 2010 groundwater elevation records for the Mexicali valley lowest (August–January) and highest (March–June) elevation periods (maps not shown) were used in conjunction with terrain elevation maps to estimate the depth of the water table along the entire CRD zone. 3.4. Hydrologic model An assessment of the river stage flow rates was conducted using the open-access HEC-RAS model from the Hydrologic Engineering Center, U.S. Army Corp of Engineers. HEC-RAS allows for onedimensional steady flow, unsteady flow, sediment transport/ mobile bed computations, and water temperature modeling (Brunner, 2010). Topographic information was included in the HEC-RAS as geometric data using the interface HecGeoRas 4.3 (Ackerman, 2011) with ArcGis 9.3. Discharge rates from 30 to 200 m3 s 1, selected after the analysis of Table 1, were used to simulate the release of pulse flows. Meanwhile, the base flows were modeled using a range of discharge flows between 1 and 20 m3 s 1. 3.4.1. HEC-RAS model calibration Numerical model calibration was performed at the RB–CC reach of the river, where RB represented the northern boundary and the hydrological inflow while CC constituted both, the southern limit and the hydrological outflow. River stage data, flow discharges, and topographic sections used for calibration were taken from CONAGUA internal files (1980’s). A peak flow of 800 m3 s 1 was

used to calibrate extreme flow events while 10–20 m3 s 1 was applied to represent the low flow regime. Upstream boundary conditions were established using flow data while downstream conditions were set applying stage data, both from the same CONAGUA files (1980’s). The initial set of Manning’s roughness coefficient values (n-values) includes those from a previous hydraulic simulation (Glenn et al., 2008; Quesada, 2007), those recommend as typical n-values by Brunner (2010). This hydraulic model considers the flow entrance at the north boundary of the area and the outflow at the south boundary. Other scenarios considering lateral inflow from spillway structures have not been analyzed. 4. Results and discussion 4.1. Identification of fluvial terraces Three fluvial terraces were identified in the riparian zone of the study area (Fig. 3). The first and lowest terraces (LT) are related to abandoned meanders and are even lower than the banks of a constructed conduit built for river channel rectification and flow control called Canal Piloto. The second and intermediate terraces (MT) correlate with the terrain riparian level prevailing before the last big flow events (1980’s), when the river levees had to be fortified and elevated. MT height lies somewhere between the agricultural lands (inside the riparian corridor) and the ancient riverbanks elevation. The third and highest terraces (HT) are at the same level than the agricultural lands outside levees, which are 0.5–1.5 m higher than MT. All of these fluvial terraces are of the unpaired type (Leet et al., 1965) due to the impact of two different events. First, water was forced to flow along with a modified course constrained by levees. Second, the main river channel was rectified by the construction of Canal Piloto, which cuts different terraces levels and resulted in a river split into several channels. The unpaired terraces emplacement means that the LT are eventually located far from the actual main river course. 4.2. Soil texture and salinity Results from soil textural analysis at the three sampling depths are summarized in Table 2 and plotted in a soil texture triangle (Fig. 4) The first 3–6 m constitute the alluvial deposits from the most recent floods period (80s’) and consist primarily of sand, silt, and a minimum percentage of clay. These textural results are in accordance with those shown in a previous report (Van Der Kamp, 1973), where sands are typically associated to active channels (Fig. 4) at any monitoring depth (Table 2). Even more, an early textural zonation obtained from lithological well cuttings (Díaz-Cabrera, 2001) also correlates well with results from these analyses. However, given the sampling points distribution along the entire study area, samples in abandoned or partially abandoned channels cannot fit within this classification (Van Der Kamp, 1973). The measured pH (7.9–8.3) defines moderately alkaline soil associated with soil drainage. On the other hand, soil salinity

Table 2 Percentage of average contents of sand, silt and clay, salinity, and pH for soil samples at three different depths. Value in parenthesis is standard deviation (S.D.). GD = groundwater depth. No. of samples

163 143 106

Depth of sampling (m)

0.2 1 GD

% Average (S.D.)

Salinity (g L

Sand

Silt

Clay

58.3 (25.3) 61.4 (27.5) 59.4 (29.1)

26.6 (19.4) 23.9 (21.3) 23.9 (21.8)

15.1 (9.8) 14.8 (11.7) 16.7 (13.3)

3.6 (4.2) 2.9 (3.1) 3.1 (2.7)

1

)

pH

8.3 (0.6) 7.9 (2.1) 8.3 (0.6)

J. Ramírez-Hernández et al. / Ecological Engineering 83 (2015) 239–248

245

more active restoration efforts such as removal of T. ramosissima and P. sericea, and planting Prosopis spp. 4.3. Surface flow modeling and riparian issues Results from surface flow modeling for both, the mimic base and pulse flows, were used to estimate the extension and depth of CRD flooded areas. The outcomes of this analysis will be discussed in the following sections. 4.3.1. Mimic base flow Simulations of mimic base flow are summarized in Table 3. As shown, discharge flows up to 10 m3 s 1 resulted in flooded extensions equal or smaller than 5.6% of the total area yet, when reaching a discharge rate of 10 m3 s 1, the total width of the main channel got flooded (Fig. 5). In addition, using 20 m3 s 1 results in the largest flooded area (7.1%) with wetted perimeter up to 12,962 m, which is 29% larger than the one obtained using 10 m3 s 1. However, in order obtain all of the hydrologic benefits of base flow, flooding of the lower alluvial terraces is not essential. Therefore, 10 m3 s 1 may be enough to elevate the groundwater levels up to the rhizosphere and to maintain soil moisture in all areas close to the main channel. Fig. 4. Soil texture classification for samples to three different depths. Color of dots corresponds to sampling depth, red = 0–0.2 m; green = 1 m; blue = phreatic level. Thick lines inside triangle are textural classification according to Soil Survey Division Staff (1993). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

showed values between 2.9 and 3.6 g L 1 (Table 2) at all depths, which is in accordance with data reported for local agricultural land (Carrillo-Guerrero, 2009). This salinity values appear to be related to water irrigation returns. For instance, water salinity in the Riito drain, which is collecting irrigation returns in the southern portion of the valley, was reported in the order of 4 g L 1 in 2010 (García-Hernández et al., 2013). It is known that, in controlled and simplified greenhouse environments, the maximum salt tolerance of S. gooddingii and P. fremontii under non-water stressed conditions was 8 g L 1 (Vandersande et al., 2001). Meanwhile, the highest salt tolerance level reported by Glenn et al. (1998) was 32 g L 1 for T. ramosissima and 16 g L 1 for P. sericea. Therefore, the high soil salinity content of the riparian zone in combination with little or no surface water availability favored the establishment of T. ramosissima and P. sericea. However, the process of flushing salts from soils through flood flows creates more suitable conditions for colonization of native species (Zamora-Arroyo et al., 2001). On the other hand, the higher terraces, which will not be flooded by pulse flows, require

Fig. 5. Flood areas under different discharge flows in the Colorado River riparian corridor obtained from hydraulic modeling by HEC-RAS.

Table 3 Flooding areas and wetted perimeter (W.P.) for various discharge rates. Total area flooded is a percentage of the study area including the main channel area. Selected base and pulse flow discharge are in bold numbers. Simulated discharge flow (m3 s 1 2 4 8 10 20 30 40 50 80 100 120 200 Total area of study is 1759 ha.

1

)

W.P. (m) 6224 7015 8016 9453 10,073 12,962 16,478 20,761 25,804 45,192 58,043 70,443 117,514

W.P. changes (%)

Area flooded (ha)

Area changes (%)

Total area flooded (%)

13 14 18 7 29 27 60 24 75 28 21 67

59 67 78 92 98 125 160 200 247 438 566 687 1150

13 16 18 6 28 27 25 24 77 29 22 67

3.4 3.8 4.4 5.2 5.6 7.1 9.1 11.3 14.0 24.9 32.1 39.1 65.4

246

J. Ramírez-Hernández et al. / Ecological Engineering 83 (2015) 239–248

Usually, under semiarid conditions, base flow infiltration is the only mechanism to replenish the groundwater table. However, given the limitations of HEC-RAS, infiltration was not evaluated hence, there is not quantification of that benefit. 4.3.2. Mimic pulse flow HEC-RAS simulations of several pulse flow discharge rates were used to evaluate lateral connectivity of surface water through flooding of floodplain terraces. A flow discharge of 20 m3 s 1 was not enough to cover LT hence, larger mimic discharge rates were simulated (Table 3). Given that the geometry of terraces and natural channels is not regular, once the main channel is flooded, changes resulting from variations in discharge rates are not linear. In this case, the change rates of the flooded area decrease by 27%, 25%, and 24%, for 30, 40 and 50 m3 s 1, respectively, indicating that flow rates variation do not produce a substantial change. Nevertheless, discharge rate of 80 m3 s 1 shows a significant change of 77% from the 50 m3 s 1, wets a perimeter of 45,192 m and floods 438 ha. This area represents only one quarter of the total study zone (Fig. 5), and includes the main channel, the old or abandoned channels, and LT and MT. An even higher discharge rate of 200 m3 s 1, earlier proposed as a pulse flow (Table 1), shows a flooded area of approximately 65%. It may yet flood some HT while leading to channel migration and significant sediment transport into the riparian corridor. In addition, this high flow rates have enough energy to compromise the levees structure and to reconnect some abandoned channel, mainly those within Laguna Grande (Fig. 2). Consequently, our model results indicate that a discharge of 200 m3 s 1 has the potential to accomplish the most important pulse flow hydrological goals. Those goals are (1) the flooding of riparian areas, (2) the migration of channels, (3) flushing salts from soil at MT and HT, and (4) the creation of a new habitat by supplying nutrients and genetic material to the riparian ecosystem (Hughes et al., 2008). By means of those 200 m3 s 1 of pulse flow, it is possible to obtain a lateral connectivity between the river and the floodplain capable of flooding the LT and MT, of wetting the soil surfaces, and of raising the HT groundwater levels, without having to use much greater volumes. 4.4. Groundwater dynamics Groundwater dynamics in the CRD are governed by return flows from irrigation lands located outside the riparian corridor. On the RB–CC river section, the surface water and groundwater

relationship is very dynamic. The stream is a gaining river capturing water from the subsurface during the intensive irrigation period (March–June). During months of low irrigation rates, groundwater levels decrease and the main channel loses water to the aquifer (August–January). The volume of water applied to agricultural lands is administered by irrigation districts called “Modulos”. During 2011–2012 crop cycle, water was applied monthly in “Modulo” 8, located at the Northern levee, and 22, adjacent to the Southern levee (Fig. 2). Fig. 6 illustrates the effects of irrigation volumes on the aquifer water table. The months of high irrigation volumes correspond with coincident wheat and cotton irrigation, while the alfalfa crop is perennial. These are the top three crops produced in the Mexicali Valley and represented 89.5% of the total farmed area in these two “Modulos” during 2011–2012 crop cycle. These annual groundwater oscillations are the response to the irrigation water calendar and they are not a function of the riparian vegetation ecosystems necessities. The perennial presence of water in the riverbed 5–6 km downstream from the RB indicates that this section of the river acts as a gaining stream maintaining the vertical connectivity with the aquifer even during the low irrigation regime. This is not the case in the southern portion of the reach, where the river gains water during the intense irrigation season but loses it very fast soon after. In this section, the riverbed becomes completely dry and the vertical connectivity is interrupted for the rest of the year, which confirms that after river regulation actions, seepage from irrigation is the only significant water source for the CRD. The spatial analysis of groundwater depths under non-wetting year conditions facilitates the selection of potential areas for riparian vegetation colonization. Table 4 shows the areal extension of groundwater depths during the low (October) and high (March) irrigation seasons. Additionally, Fig. 7 displays the spatial distribution of those groundwater depths for both seasons. The total area which depth-to-groundwater is less than 2 m extends along 526 and 249 ha for each, the high and the low irrigation seasons, respectively. All of this area corresponds to the LT level and has the potential to be revegetated with S. gooddingii and P. fremontii. Other areas with groundwater depths between 3.1 and 4 m are associated to MT and cover 586–609 ha under high and low irrigation regimes, respectively (Table 4). Both S. gooddingii and P. fremontii species could be established in this area but, due to the P. fremontii species capacity to reach deeper groundwater levels, this will be more likely the one to colonize the MT’s. For areas that along both irrigation seasons keep up to 6 m depth-to-groundwater, revegetation with Prosopis spp. is recommended given that

Fig. 6. Volume of water used for irrigation during crop cycle 2011–2012 in Modulos 8 and 22 located on both sides of study area. Irrigation data from CONAGUA (2013). Water table oscillation of piezometer located inside “CILA restoration site”. Data from Sonoran Institute.

J. Ramírez-Hernández et al. / Ecological Engineering 83 (2015) 239–248

247

Table 4 Areas of groundwater depth for both irrigation seasons. Maximum recharge from irrigation lands for March and minimum recharge for October. Main channel area was not considered for percentage of study area and cumulative percentage. Data from groundwater elevation was compiled for 2010 year. GW depth (m)

<0.01 0.01–1 1.1–2 2.1–3 3.1–4 4.1–5 5.1–6 6.1–7 >7.1

March

October

Area (ha)

Area (%)

Cumulative (%)

Area (ha)

Area (%)

Cumulative (%)

6.9 45.0 179.7 525.9 585.9 224.7 50.2 10.4 0.2

0.4 2.6 10.2 29.9 33.3 12.8 2.9 0.6 0.01

0.4 2.9 13.2 43.1 76.3 89.1 92.0 92.6 92.6

3.5 11.9 99.6 249.0 609.0 431.1 177.2 41.2 8.2

0.2 0.7 5.7 14.2 34.6 24.5 10.1 2.3 0.5

0.2 0.9 6.5 20.7 55.3 79.8 89.9 92.2 92.7

enough to flood the MT and a portion of the HT (65.4% of total CRD area). Still, this discharge rate may represent a risk to levees and to irrigation lands outside the riparian corridor. Additionally, pulse flows must be scheduled in late spring to overlap with the native vegetation seeding season and the high groundwater levels (caused by intense irrigation), to increase the sprouting probability of native species like P. fremontii and S. gooddingii. (3) Depth-to-groundwater maps elaborated from field data for both, the higher and lower irrigation regimes during the 2011–2012 season, show that at least 55% of the study area is suitable for riparian restoration with species such as S. gooddingii and P. fremontii due to the fact that groundwater is less than 4 m depth over the entire year. On the other hand, groundwater levels deeper than 5 m, which place these water resources out of the reach of most riparian vegetation, extend for only for 2.8% and 0.6% of the study area under low and high irrigation conditions, respectively. (4) This kind of hydrological analysis can be applied to other reaches of the Colorado River to identify potential restoration areas for groundwater dependent ecosystems.

Fig. 7. Groundwater depth distribution for high and low irrigation seasons, March and October, respectively.

they are able to utilize deep groundwater and shallow soil moisture. More than 55% of the study area maintains groundwater depths equal or lesser than 4 m (Table 4). This is a very significant percentage considering that this is the maximum depth-togroundwater at which S. gooddingii and P. fremontii can be established as a dense and multi-aged forest (Lite and Stromberg, 2005). 5. Conclusions (1) Our modeling results indicate that a release of 10 m3 s 1 is suitable to increase the surface available for native riparian hydrophytes colonization. This discharge rate is essential to make the most of all the benefits of an adequate base flow, such as maintaining flooded surfaces and permanent water bodies, and connecting old channels to the mainstream. Given that timing and length of the growing season for riparian vegetation are independent of the irrigation cycles, in order to replenish the groundwater levels, controlled water releases that mimic base flow must be scheduled during periods of lower irrigation. (2) According to the mimic pulse flow models, discharge rates below 80 m3 s 1 on the RB–CC section will likely not be sufficient to inundate more than 25% of the CRD area, neither to accomplish any of the hydrological processes linked to a pulse flow. On the other hand, a pulse flow of 200 m3 s 1 will be

Acknowledgements We acknowledge the National Waters Commission (CONAGUA), especially Francisco Telles, Jesús Flores y Carlos Robles VanDyck for sharing hydrologic data as well as allowing access to their reports. This work was funded by Higher Education for Development, U.S. Agency for International Development (USAID) 2006–2008, the Autonomous University of Baja California (UABC-DGIYP/10a Convocatoria), Sonoran Institute, and Pronatura Noroeste. We gratefully acknowledge all the students at UABC for their enthusiastic participation in collecting and analyzing data. We appreciate discussions with E. Santiago, T. Rivas and O. Hinojosa that contributed to this manuscript and English translation by Karen Schlatter and Fatima Luna. We also greatly appreciate the field staff that helped on this project. References Ackerman, C.T., 2011. HEC-GeoRAS GIS Tools for Support of HEC-RAS Using Arcgis, User’s Manual. US Army Corps of Engineers, Institute for Water Resources Hydrologic Engineering Center (HEC) Available online: http://www.hec.usace. army.mil/software/hec-ras/documents/HEC-GeoRAS_43_Users_Manual.pdf. Ariel Construcciones, S.A., 1968. Estudio Hidrogeológico preliminar de los Acuíferos del Valle de Mexicali, BC. y Mesa Arenosa de San Luis Sonora. CONAGUA, Mexico. ASTM-D422, 1963. Standard Test Method for particle—Size Analysis of Soils. American Society for Testing and Materials Method (reapproved 1998). Bouyucos, G.J., 1936. Directions for making mechanical analyses of soils by the hydrometer method. Soil Sci. 42, 225–230. Briggs, M., Cornelius, S., 1998. Opportunities for ecological improvement along the lower Colorado River and delta. Wetlands 18, 513–529.

248

J. Ramírez-Hernández et al. / Ecological Engineering 83 (2015) 239–248

Brunner, G., 2010. HEC-RAS, River Analysis System User’s Manual. U.S. Army Corps of Engineers, Institute of Water Resources, Hydrologic Engineering Center Available online: http://www.hec.usace.army.mil/software/hec-ras/. Busch, D.E., Smith, S.D., 1995. Mechanisms associated with decline of woody species in riparian ecosystems of the southwestern U.S. Ecol. Monogr. 65, 347–370. Carrillo-Guerrero, Y.K., 2009. Water Conservation, Wetland Restoration and Agriculture in the Colorado River Delta, Mexico, Ph.D.. The University of Arizona, Tucson, AZ. CONAGUA, 2007. Reporte de niveles y caudales en el Puente del Ferrocarril MexicaliPuerto Peñasco con el Río Colorado en los escurrimientos de 1993–1994. CONAGUA. CONAGUA, 2012. Datos Climatológicos De Baja California. Gerencia Regional de la Peninsula de Baja California, Subgerencia Regional Tecnica, Mexicali, B.C., Mexico (unpublished data). CONAGUA, 2013. Reporte de Volumenes de Irrigación por Módulos del Distrito de Riego 014 de Baja California. Gerencia Regional de la Peninsula de Baja California, Subgerencia Regional Técnica, Mexicali, B.C., Mexico (unpublished data). Cupul-Magaña, G., 2009. Clasificación de la vegetación y su correlación con la profundidad del nivel freático en la vega del Río Colorado en el Tramo FFCCVado Carranza, México, Master Eng.. Universidad Autonoma de Baja California, Mexicali, B.C. Díaz-Cabrera, P., 2001. Simulación Numérica del acuífero superior del Valle de Mexicali, Baja California, México, Master Sc.. Centro de Investigación Científica y de Educación Superior de Ensenada, Ensenada, B.C. Dickinson, J.E., Land, M., Faunt, C.C., Leake, S.A., Reichard, E.G., Fleming, J.B., Pool, D. R., 2006. Hydrologic Framework Refinement, Ground-water Flow and Storage, Water-chemistry Analyses, and Water-budget Components of the Yuma Area, Southwestern Arizona and Southeastern California, Reston, Virginia. U.S. GEOLOGICAL SURVEY Scientific Investigations Report 2006–5135, http://pubs. usgs.gov/sir/2006/5135/ Elders, W.A., Hoagland, J.R., McDowell, S.D., Cobo, J.M., 1979. Hydrothermal mineral zones in the geothermal reservoir of Cerro Prieto. Geothermics 8, 201–209. ESRI, 2006. ArcMap 9.2. Environmental Systems Research Institute, Redlands, California, USA. Fradkin, P.L., 1981. A River No More: The Colorado River and the West, second ed. University of California Press, Berkeley. García-Hernández, J., Flessa, K., Santiago-Serrano, E., Romero-Hernández, S., Zamora-Arroyo, F., Ramírez-Hernández, J., 2013. Salinity responses to inflow alterations in a 6500 ha Typha wetland. Ecol. Eng. 52, 191–202. Getches, D.H., 2003. Water management in the United States and the fate of the Colorado River Delta in Mexico. U. S. Mex. Law J. 11, 107–113. Glenn, E.P., Lee, C., Felger, R., Zengel, S., 1996. Effects of water management on the wetlands of the Colorado River delta, Mexico. Conserv. Biol. 10, 1175–1186. Glenn, E., Tanner, R., Mendez, S., Kehret, T., Moore, D., Garcia, J., Valdes, C., 1998. Growth rates, salt tolerance and water use characteristics of native and invasive riparian plants from the delta of the Colorado River, Mexico. J. Arid Environ. 40, 281–294. Glenn, E.P., Hucklebridge, K., Hinojosa-Huerta, O., Nagler, P.L., Pitt, J., 2008. Reconciling environmental and flood control goals on an arid-zone river: case study of the limitrophe region of the Lower Colorado River in the United States and Mexico. Environ. Manag. 41, 322–335. Hughes, F.M.R., Moss, T., Richards, K.S., 2008. Uncertainty in riparian and floodplain restoration. In: Darby, S., Sear, D. (Eds.), River Restoration. Managing the Uncertainty in Restoration Physical Habitat. John Wiley and Sons, West Sussex, England, pp. 79–104. Hughes, F.M.R., Rood, S.B., 2003. ALlocation of river flows for restoration of floodplain forest ecosystems: a review of approaches and their applicability in Europe. Environ. Manag. 32, 12–33. IBWC, 2000. Minute no. 306-Conceptual framework for U.S. – Mx studies for future recommendations concerning the riparian and estuarine ecology of the limitrophe section of the Colorado River and its associated delta. In: I.B.W.C., http://www.ibwc.gov/Files/Minutes/Min306.pdf. IBWC, 2010a. Minute no. 316-Utilization of the Wellton–Mohawk bypass drain and necessary infrastructure in the United States for the conveyance of water by Mexico and non-governmental organizations of both countries to the Santa Clara wetland during the Yuma Desalting Plant pilot run. In: I.B.W.C., http:// www.ibwc.gov/Files/Minutes/Min_316.pdf. IBWC, 2010b. Minute no. 317-Conceptual framework for U.S.–Mexico discussions on Colorado River cooperative actions. In: I.B.W.C., http://www.ibwc.gov/Files/ Minutes/Min_317.pdf.

IBWC, 2010c. Minute no. 318-Adjustment of delivery schedules for water allotted to Mexico for the years 2010 through 2013 as a result of infrastructure damage in irrigation district 014, Rio Colorado, caused by the April 2010 earthquake in the Mexicali Valley, Baja California. In: I.B.W.C., http://www.ibwc.state.gov/Files/ Minutes/Min_318.pdf. IBWC, 2012. Minute no. 319-Interim international cooperative measures in the Colorado River Basin through 2017 and extension of Minute 318 cooperative measures to address the continued effects of the April 2010 earthquake in the Mexicali Valley, Baja California. In: I.B.W.C., http://www.ibwc.state.gov/Files/ Minutes/Minute_319.pdf. Junk, W.J., Baykey, P.B., Sparks, R.E., 1989. The flood pulse concept in river-floodplain systems. Can. Spec. Publ. Fischeries Aquat. Sci. 106, 110–127. Leet, L.D., Judson, S., Schmitz, E.A., 1965. Physical Geology, third ed. Prentice-Hall, New Jersey. Lippmann, M.J., Truesdell, A.H., Halfman-Dooley, S.E., Mañónm, A., 1991. A review of the hydrogeologic-geochemical model for Cerro Prieto. Geothermics 20, 39–52. Lite, S.J., Stromberg, J.C., 2005. Surface water and ground-water thresholds for maintaining Populus–Salix forests, San Pedro River, Arizona. Biol. Conserv. 125, 153–167. Luecke, D., Pitt, J., Congdon, C., Glenn, E., Valdes-Casillas, C., Briggs, M., 1999. A Delta Once More: Restoring Riparian and Wetland Habitat in the Colorado River Delta. Environmental Defense Fund, Washington DC, USA. Meckel, L.D., 1975. Holocene sand bodies in the Colorado delta area, northern Gulf of California. In: Broussard, M.C. (Ed.), Deltas, Models for Exploration. Huston Geological Society, Texas, U.S, pp. 239–265. Ohmar, D.R., Deason, W.O., Burke, C., 1977. A riparian case history: the Colorado River. In: Johnson, R.R., Jones, D.A. (Eds.), Preservation and Management of Riparian Habitat. U.S. Department of Agriculture/Forest Service. Ohmart, R., Anderson, B., Hunter, W., 1988. Ecology of the Lower Colorado River from Davis Dam to the Mexico–United States Boundary: A Community Profile National Technical Information Service, Alexandria, VA. Olmsted, F.H., Loeltz, O.J., Irelan, B., 1973. Geohydrology of the Yuma Area, Arizona and California. Water Resources of Lower Colorado River–Salton Sea Area. U. S. Department of the Interior G.S., Washington, D.C., Estado Unidos de America. Poff, N.L., Allan, J.D., Bain, M.B., Karr, J.R., Prestegaard, K.L., Richter, B.D., Sparks, R.E., Stromberg, J.C., 1997. The Natural Flow Regime. Bioscience 47, 769–784. Quesada, A.N., 2007. Simulación hidráulica del Río Colorado en el tramo puente ferroviario-vado Carranza, Mexicali, B.C, México, Master Eng., Universidad Autónoma de Baja California, Mexicali, B.C. Ramírez-Hernández, J., Hinojosa-Huerta, O., Peregrina-Llanes, M., Calvo-Fonseca, A., Carrera-Villa, E., 2013. Groundwater responses to controlled water releases in the limitrophe region of the Colorado River: implications for management and restoration. Ecol. Eng. 59, 93–103. doi:http://dx.doi.org/10.1016/j. ecoleng.2013.02.016. Soil Survey Division Staff, 1993. Soil survey manual. Soil Conservation Service, 18. U. S. Department of Agriculture Handbook. Stockton, C.W., Jacoby, G., 1976. Long-term surface-water supply and streamflow trends in the Upper Colorado River Basin. Available from the National Technical Information Service, Springfield VA 22161 as PB-264 533, Price codes: A 05 in paper copy, A 01 in microfiche. Lake Powell Research Project Bulletin. Stromberg, J.C., 1998. Dynamics of Fremont cottonwood (Populus fremontii) and saltcedar (Tamarix chinensis) populations along the San Pedro River, Arizona. J. Arid Environ. 40, 133–155. Stromberg, J.C., 2001. Restoration of riparian vegetation in the south-western United States: importance of flow regimes and fluvial dynamism. J. Arid Environ. 49, 17–34. Sykes, G., 1937. The Colorado Delta. ed. Carnegie Institution, Washington, D.C. Publication Number 460. Tetra Tech, I., 2004. Lower Colorado River, Final Report, Proposed Pilot Channel Analysis. Prepared for U.S. Army Corps of Engineers, Los Angeles District. Van Der Kamp, P.C., 1973. Holocene continental sedimentation in the Salton basin, California: a reconnaissance. Geol. Soc. Am. Bull. 22. Vandersande, M.W., Glenn, E.P., Walworth, J.L., 2001. Tolerance of five riparian plants from the lower Colorado River to salinity drought and inundation. J. Arid Environ. 49, 147–159. Zamora-Arroyo, F., Nagler, P.L., Briggs, M., Radtke, D., Rodriquez, H., Garcia, J., Valdes, C., Huete, A., Glenn, E.P., 2001. Regeneration of native trees in response to flood releases from the United States into the delta of the Colorado River, Mexico. J. Arid Environ. 49, 49–64.