Rates Of Water Table Decline Research Articles

Putah Creek hydrology affecting riparian cottonwood and willow tree survival.

Creating or recruiting new riparian forests to improve Lower Putah Creek (LPC) ecosystem functions is challenging under the modified stream flow regime developed after historic gravel mining and installation of Monticello Dam upstream. Hydrologic connectivity between riparian trees, shallow groundwater, and the low flow channel is essential towards maintaining these forests and related habitats through the annual summer and multi-year drought periods typical in this Sacramento Valley region of California. Despite increased average summer flows, significant mature cottonwood and willow tree mortality along the LPC riparian areas below the Putah Creek diversion dam in 2014 raised concerns over the soil and hydrologic factors affecting riparian vegetation survival. A forensic analysis was conducted combining annual canopy coverage fractions and tree ring studies with daily soil-water balances, low flow records, and available groundwater level Information from the past few decades to determine the key hydrologic factors affecting riparian tree survival along the LPC. The 2011-2016 drought was linked with greater than prior average soil-water deficits in 2012-2015 and lower initial soil-water storage on March 1 of 2012 and 2014 that would be expected to stress the trees. However, such stress was not apparent in decreased tree ring spacing during this period from mature (40-50years old), deceased, and living trees. Tree canopy coverage declined dramatically (by as much as 50% as compared to the previous decade average) only in the summer of the 2014 despite a ~ 35% increase in average summer flows from 2011 to 2014. However, the regional water table aquifer levels declined at an average rate of ~ 35mm/day in 2014 (as compared to ~ 17mm/day in previous decade) and by several meters overall between 2011 and 2016 suggesting that deceased trees lacked access to the water table aquifer or lateral stream seepage. The increased rates of water table decline and overall depth may be associated with a large increase in adjacent irrigated almond orchard areas in 2014-2016. Knowledge of the dynamic hydrologic factors controlling sustainability of riparian trees should better inform and guide future tree restoration efforts along the LPC.

Map visualization of groundwater withdrawals at the sub-basin scale

A simple method is proposed to visualize the magnitude of groundwater withdrawals from wells relative to user-defined water-resource metrics. The map is solely an illustration of the withdrawal magnitudes, spatially centered on wells—it is not capture zones or source areas contributing recharge to wells. Common practice is to scale the size (area) of withdrawal well symbols proportional to pumping rate. Symbols are drawn large enough to be visible, but not so large that they overlap excessively. In contrast to such graphics-based symbol sizes, the proposed method uses a depth-rate index (length per time) to visualize the well withdrawal rates by volumetrically consistent areas, called “footprints”. The area of each individual well’s footprint is the withdrawal rate divided by the depth-rate index. For example, the groundwater recharge rate could be used as a depth-rate index to show how large withdrawals are relative to that recharge. To account for the interference of nearby wells, composite footprints are computed by iterative nearest-neighbor distribution of excess withdrawals on a computational and display grid having uniform square cells. The map shows circular footprints at individual isolated wells and merged footprint areas where wells’ individual footprints overlap. Examples are presented for depth-rate indexes corresponding to recharge, to spatially variable stream baseflow (normalized by basin area), and to the average rate of water-table decline (scaled by specific yield). These depth-rate indexes are water-resource metrics, and the footprints visualize the magnitude of withdrawals relative to these metrics.

Groundwater‐dependent vegetation: Quantifying the groundwater subsidy

The typical stratigraphy of riparian ecosystems consists of fine‐grained overbank deposits overlying coarser‐grained materials. Plants within these regions rely on soil moisture in the fine‐grained sediments as well as supplemental groundwater for root water uptake. The additional water available as a result of shallow water table conditions is defined here as groundwater subsidy and is found to be a significant contribution to root water uptake. Work presented here quantifies the effect of groundwater subsidy on root water uptake as a result of variations in the soil thickness of the upper fine‐grained sediments, rate of water table decline, and maximum water table depth. Variations in soil thickness and water table decline regimes produce a complex response with respect to both the rate of groundwater subsidy and the cumulative groundwater subsidy. These simulated regimes are analogs to environmental scenarios in riparian ecosystems that result from stream incision, soil erosion, and climate change. These results have implications for identifying ecosystems most susceptible to future change as well as those most amenable to restoration.

A method for estimating subdaily evapotranspiration of shallow groundwater using diurnal water table fluctuations

AbstractDiurnal water table fluctuations are a common feature of well hydrographs recorded in wetlands, riparian areas, and similar environments where shallow groundwater supports phreatophytic vegetation. Historically, this periodic signal has been used to estimate daily groundwater consumption by this vegetation and has shown that this is typically a very large component of the water budget. As interest in ecohydrology grows and the need for finer resolution estimates of groundwater consumption increases, new, cost‐effective measurement methodologies must be developed. Here, a method is proposed that uses the observed rate of water table decline during the day to estimate phreatophytic groundwater consumption while using the nighttime records to account for other groundwater fluxes to/from the vicinity of the well at all times. Variably saturated groundwater flow modeling was used to create a synthetic data set to test the methodology, and results showed that accurate, sub‐daily (15 min) estimates of phreatophytic groundwater consumption were obtained. The method was also applied to a preexisting data set to demonstrate the usefulness of the technique. Copyright © 2008 John Wiley & Sons, Ltd.

Seedling growth of Acacia tortilis and Faidherbia albida in response to simulated groundwater tables

The growth and survival of woody seedlings in arid and semi-arid riverine forests is partly determined by the ability of their roots to maintain contact with the alluvial water table. After a flood event, the water table declines in conjunction with the river stage and the rate of decline may have species-specific effects on the regeneration of riverine trees. In this study, a growth experiment involving seedlings of Acacia tortilis and Faidherbia albida, two keystone tree species along the regulated Turkwel River in northern Kenya, was conducted to assess the effects of simulated pre- and post-dam water table dynamics. Seedlings were grown in rhizopods with constantly shallow water tables (0 cm day −1 treatment), slow rates of water table decline (5 cm day −1 treatment), rapid rates of water table decline (10 cm day −1 treatment) and no subsurface water (rainfall treatment). The last treatment corresponded to the dam-induced paucity of floodplain inundation but allowed early seedling growth in response to rainfall. Results showed that F. albida attained larger shoot growth but shorter root lengths than A. tortilis, while water table decline promoted root elongation in both species. However, F. albida seedlings were adversely affected by moisture deficits under the rapid rate of water table decline and rainfall treatments. In contrast, the growth of A. tortilis seedlings was sustained under all treatments, suggesting that A. tortilis is a drought tolerant facultative phreatophyte, while F. albida behaves more like a near obligate phreatophyte. For F. albida, the ideal stage recession rate following a flood event is 5 cm day −1 or less, which is also favourable for A. tortilis. Although more research is still needed on the hydrological preferences of woody seedlings in the Turkwel River floodplain, it is evident that the regeneration of strictly riverine trees such as F. albida depends on slow rates of water table decline in the post-flood period.

Survival and growth responses of Populus nigra, Salix elaeagnos and Alnus incana cuttings to varying levels of hydric stress

The Salicaceae and Betulaceae are important plant families within riparian ecosystems throughout the Northern hemisphere, and cuttings of species within these families are often used in riparian restoration projects due to their capacity for vegetative (asexual) propagation and rapid growth. Riparian zones can experience a wide range of hydrological conditions and establishing cuttings may be subjected to substantially varying levels of hydric stress from inundation and drought. Tolerance to such stress can vary notably between species, and the absence of knowledge of species-specific responses to inundation and drought has resulted in high mortality in many riparian restoration projects. In this paper, the survival and growth responses of Salix elaeagnos (Salicaceae), Populus nigra (Salicaceae) and Alnus incana (Betulaceae) cuttings in relation to varying rates of water table decline and periods of inundation were investigated using rhizopods during a 69-day greenhouse experiment, in substrates of contrasting sediments (sand versus gravel). Each species responded differently to the experimental treatments. S. elaeagnos demonstrated an ability to establish in stable, declining and inundated conditions by the production of relatively substantial root and shoot biomass in all treatments, along with elongated roots when experiencing water table decline. P. nigra showed high mortality and poor root and shoot production in inundation treatments, suggesting that this species is intolerant to reduced oxygen conditions. A. incana displayed complete mortality in all gravel-based treatments and high mortality or poor growth in the sand-based inundation treatments. All species showed preferences for stable conditions without inundation, along with slow water table decline for S. elaeagnos. Fast rates of water table decline were detrimental to all species. Overall, sand was a much more effective substrate than gravel for supporting cutting establishment. These results suggest that riparian restoration schemes utilizing S. elaeagnos cuttings may experience greater success than those using the other species investigated. P. nigra cuttings may establish more easily if situated where inundation does not occur often or for prolonged periods, and A. incana cuttings should be planted only where very stable conditions dominate. The study highlights the inter-species variability in the survival and growth response of riparian trees to differing hydrological conditions.

Water table decline alters growth and survival of Salix gooddingii and Tamarix chinensis seedlings

In the western United States, many native riparian forests are declining downstream from dams and diversions, primarily due to a lack of successful regeneration of native species. Conceptual models have been described that incorporate stream flow characteristics that promote successful regeneration of western riparian Populus seedlings. However, these models need to be calibrated for additional species and specific stream characteristics. In this study, we examined the growth and survival of two dominant Sonoran desert riparian tree species, a native Salix gooddingii, and an exotic, Tamarix chinensis, to simulated water table declines at rates of 0, 1, 2, and 4 cm/day. Salix seedlings performed best in the 0 cm/day treatment in which the soil was saturated and showed decreasing survival and growth with increasing rates of water table decline. Root-to-shoot ratios were high for Salix, but root elongation rates were low with an emphasis on lateral root development. This root growth pattern may be an adaptation to habitat where Salix typically grows, near the stream channel where the water table is shallow but the likelihood of flood scour is high. Lateral root development may help to protect Salix seedlings from late season flood scour. Tamarix seedlings showed consistently high survival across all treatments and had the greatest growth in the 0 and 1 cm/day treatments. Tamarix seedlings had greater root elongation rates in response to water table decline relative to the control, with the greatest elongation in the 1 cm/day treatment. Greater root elongation rates and higher drought tolerance allowed Tamarix seedlings to persist in dry soils where Salix seedlings died. Differences in growth and survival of these two species in response to water table decline will need to be incorporated into management practices to promote successful native riparian tree regeneration.

The responses of three riparian cottonwood species to water table decline

The influence of rate of water table decline was studied with three North American cottonwood (poplar) species: the prairie cottonwood, Populus deltoides; the narrowleaf cottonwood, P. angustifolia; and the balsam poplar, P. balsamifera. Shoot cuttings were rooted and transplanted into rhizopods, experimental devices that permit the controlled manipulation of water table depth. Three rates of water table decline were applied, 0, 4 and 10 cm day −1, and growth and transpiration were studied. Two clones of each species performed relatively similarly; the P. balsamifera clones grew fastest under all three treatments, followed by P. deltoides under 0 and 4 cm day −1 conditions. Under the 10 cm day −1 treatment, the P. deltoides grew as slowly as P. angustifolia. In all genotypes, shoot growth and apparent transpiration were progressively reduced with increasing rate of water table decline. Conversely, root growth was promoted by water table decline and root elongation was most rapid under the gradual 4 cm day −1 treatment; root elongation was insufficient for the abrupt 10 cm day −1 decline and some P. angustifolia and P. deltoides saplings died under that treatment. The present study demonstrates that tolerance to water table decline varies across cottonwood genotypes and that P. balsamifera saplings were the most vigorous. This is relevant to the natural distribution in which P. balsamifera occurs in mountain regions where stream stages and riparian water table depths often change abruptly. The vigor of P. balsamifera is also consistent with the reproductive mechanism of `branch propagation', a process of clonal recruitment in which browsed or broken branch fragments root along stream edges, enabling dispersive propagation, particularly of P. balsamifera and P. trichocarpa.

Response of a hybrid poplar to water table decline in different substrates

The effects of substrate texture and rate of water table decline on the growth of a Populus balsamifera x Populus deltoides hybrid were studied using rhizopods. Rooted shoot cuttings were transplanted into growth tubes filled with gravel, sand, or a 1:1 (v/v) mixture of sand and were treated with water level declines of 0, 2, 5 or 10 cm day−1.Water drained fastest from the gravel-filled tubes, intermediate in the mixture and slowest from the sand-filled tubes. The effects of rapid water table decline were most severe on plants grown in the gravel and least severe on those grown in sand. Transpiration, height, leaf number, leaf area and plant health decreased with increasing rates of water table decline and increasing gravel content in the substrate. The reduction in transpiration and plant growth indicates that rapid water table decline caused drought stress of the poplars. Root elongation was promoted in all substrates by water table decline. The results indicate that alterations to river flow that cause abrupt drops in riparian water table will retard the transpiration and growth of riparian poplars. These effects will be more severe along flood-plains with coarse substrates.

Vertical Leakage in Egypt's Nile Valleys Estimation and Implications

The determination of vertical ground‐water leakage is particularly important for irrigated agriculture where adequate drainage is required to prevent high water tables and related problems of salt buildup and waterlogging. This paper presents results from six different methods that were used to estimate vertical leakage for the Nile Valley in Egypt: (1) Calculation by Darcy's law; (2) rate of water table decline; (3) water budgets; (4) leaky aquifer test; (5) analytical solution; and (6) numerical ground‐water modeling. Considering the basic differences in the underlying assumptions of each method, the results were in relatively good agreement. The average vertical leakage determined for the entire Nile Valley was about 0.65ċ10-3m/day. This value is quite low, much lower than previously reported elsewhere in the literature. This low rate of natural drainage results in high water tables that cannot be lowered without the use of artificial drains. A salt concentration balance indicated that given the small value of vertical leakage in the absence of artificial drainage, the salinity of the ground water in the Nile Valley may increase to the point where it would hinder crop production.

A device for studying the influence of declining water table on poplar growth and survival.

Riparian poplar forests are declining downstream from many dams in the western prairie regions of North America. Drought-induced seedling and sapling mortality caused by abrupt drops in water table level following river flow regulation may be contributing to this forest decline. Here we describe a device, the rhizopod, for studying the influence of a changing water table on poplar seedling survival and growth. Seeds of a natural poplar hybrid (Populus deltoides x P. balsamifera) were planted in five rhizopods and treated to water table declines of 0, 1, 2, 4, or 8 cm day(-1). Each rhizopod consisted of 15 growth tubes connected to a central water reservoir through which the water level was regulated. Seedling survival was over 90% in rhizopods in which water table declined by 0, 1 or 2 cm day(-1), but was reduced to about 40% and less than 25% in rhizopods in which the water table level declined by 4 and 8 cm day(-1), respectively. Maximum shoot height, leaf number, and leaf area were observed in the rhizopod with a constant water table and decreased progressively with increasing rates of water table decline. Maximum root mass and length were observed in plants subjected to declines in water table level of 0 and 1 cm day(-1), respectively.