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1

Quantifying the surface-subsurface biogeochemical coupling during the VERTIGO ALOHA and K2 studies

A central question addressed by the VERTIGO (VERtical Transport In the Global Ocean) study was 'What controls the efficiency of particle export between the surface and subsurface ocean'? Here, we present data from sites at ALOHA (N Central Pacific Gyre) and K2 (NW subarctic Pacific) on phytoplankton processes, and relate them via a simple planktonic foodweb model, to subsurface particle export (150-500 m). Three key factors enable quantification of the surface-subsurface coupling: a sampling design to overcome the temporal lag and spatial displacement between surface and subsurface processes; data on the size-partitioning of Net Primary Production (NPP) and subsequent transformations prior to export; estimates of the ratio of algal- to faecal-mediated vertical export flux. At ALOHA, phytoplankton were characterized by low stocks, NPP, F{sub v}/F{sub m} (N-limited), and were dominated by picoplankton. The HNLC waters at K2 were characterized by both two-fold changes in NPP and floristic shifts (high to low proportion of diatoms) between deployment 1 and 2. Prediction of export exiting the euphotic zone was based on size-partitioning of NPP, a copepod-dominated foodweb and a ratio of 0.2 (ALOHA) and 0.1 (K2) for algal:faecal particle flux. Predicted export was 20-22 mg POC m{sup -2} d{sup -1} at ALOHA (i.e. 10-11% NPP (0-125 m); 1.1-1.2 x export flux at 150 m (E{sub 150}). At K2, export was 111 mg C m{sup -2} d{sup -1} (21% NPP (0-50 m); 1.8 x E{sub 150}) and 33 mg POC m{sup -2} d{sup -1} (11% NPP, 0-55 m); 1.4 x E{sub 150}) for deployments 1 and 2, respectively. This decrease in predicted export at K2 matches the observed trend for E{sub 150}. Also, the low attenuation of export flux from 60 to 150 m is consistent with that between 150 to 500 m. This strong surface-subsurface coupling suggests that phytoplankton productivity and floristics play a key role at K2 in setting export flux, and moreover that pelagic particle transformations by grazers strongly influence to what extent sinking particles are further broken down in the underlying waters of the Twilight Zone.

Boyd, P.W.; Gall, M.P.; Silver, M.W.; Bishop, J.K.B.; Coale, Susan L.; Bidigare, Robert R.

2008-02-25

2

A Coupled Surface/Subsurface Model for Hydrological Drought Investigations

NASA Astrophysics Data System (ADS)

Hydrological droughts occur when storage in the ground and surface-water bodies falls below statistical average. Due to the inclusion of regional groundwater, hydrological droughts evolve relatively slowly. The atmospheric and surface components of the hydrological cycle have been widely studied, are well understood, and their prognoses are fairly accurate. In large-scale land surface models on the other hand, subsurface (groundwater) flow processes are usually assumed unidirectional and limited to the vertically-downward percolation and the horizontal runoffs. The vertical feedback from groundwater to the unsaturated zone as well as the groundwater recharge from surface waters are usually misrepresented, resulting in poor model performance during low-flow periods. The feedback is important during meteorological droughts because it replenishes soil moisture from ground- and surface water, thereby delaying the onset of agricultural droughts. If sustained for long periods however, the depletion can significantly reduce surface and subsurface storage and lead to severe hydrological droughts. We hypothesise that an explicit incorporation of the groundwater component into an existing land surface model would lead to better representation of low flows, which is critical for drought analyses. It would also improve the model performance during low-flow periods. For this purpose, we coupled the process-based mHM surface model (Samaniego et al. 2010) with MODFLOW (Harbaugh 2005) to analyse droughts in the Unstrut catchment, one of the tributaries of the Elbe. The catchment is located in one of the most drought-prone areas of Germany. We present results for stand-alone and coupled mHM simulations for the period 1970-2000. References Arlen W. Harbaugh. MODFLOW-2005, The U.S. Geological Survey Modular Ground-water Model-the Ground-water Flow Process, chapter Modelling techniques, sec. A. Ground water, pages 1:1-9:62. USGS, 2005. Luis Samaniego, Rohini Kumar, and Sabine Attinger. Multiscale parameter regionalization of a grid-based hydrologic model at the mesoscale. Water Resour. Res., 46(W05523), 2010. doi: 10.1029/2008WR007327.

Musuuza, J. L.; Kumar, R.; Samaniego, L. E.; Fischer, T.; Kolditz, O.; Attinger, S.

2013-12-01

3

NASA Astrophysics Data System (ADS)

The ensemble Kalman filter (EnKF) and sequential importance resampling (SIR) are two Monte Carlo-based sequential data assimilation (DA) methods developed to solve the filtering problem in nonlinear systems. Both methods present drawbacks when applied to physically-based nonlinear models: the EnKF update is affected by the inherent Gaussian approximation, while SIR may require a large number of Monte Carlo realizations to ensure consistent updates. In this work we implemented EnKF and SIR into a physically-based coupled surface-subsurface flow model and applied it to a synthetic test case that considers a uniform soil v-shaped catchment subject to rainfall and evaporation events. After a sensitivity analysis on the number of Monte Carlo realizations and the correlation time of the atmospheric forcing, the comparison between the two filters is done on the basis of different simulation scenarios varying observations (outlet streamflow and/or pressure head), assimilation frequency, and type of bias (atmospheric forcing or initial conditions). The results demonstrate that both EnKF and SIR are suitable DA methods for detailed physically-based hydrological modeling using the same, relatively small, ensemble size. We highlight that the Gaussian approximation in the EnKF updates leads to a state estimation that can be not consistent with the physics of the model, resulting in a slowdown of the numerical solver. SIR instead duplicates physically consistent realizations, but can display difficulties in updates when the realizations are far from the true state. We propose and test a modification of the SIR algorithm to overcome this issue and preserve assimilation efficiency.

Pasetto, Damiano; Camporese, Matteo; Putti, Mario

2012-10-01

4

NASA Astrophysics Data System (ADS)

Coastal aquifers are complex hydrologic systems because many physical processes interact: (i) variably saturated flow, (ii) spatial-temporal fluid density variations, (iii) tidal fluctuations, (iv) storm surges overtopping dykes, and (v) surface runoff of storm water. The HydroGeoSphere model is used to numerically simulate coastal flow dynamics, assuming a fully coupled surface-subsurface approach, accounting for all processes listed above. The diffusive wave approximation of the St. Venant equation is used to describe surface flow. Surface flow and salt transport are fully coupled with subsurfacial variably saturated, variable-density flow and salt transport through mathematical terms that represent exchange of fluid mass and solute mass, respectively. Tides and storm surges induce a time-variant head that is applied to nodes of the surface domain. The approach is applied to real cases of tide and storm surge events of the coastal catchment area of Unterweser, Northern Germany (ca. 2500 km2). To optimize the simulation and to reduce CPU cost, the actual simulation domain is only a part of the total catchment area: We selected a narrow strip of about 10 km width parallel to the coastline. The catchment area outside of that strip is not affected by saltwater intrusion and does therefore not need to be included in the numerical model. The seaside boundary condition of the reduced simulation domain is obtained from a 3D hydrodynamic-numerical flow model where changes in sea level are being simulated. The landside boundary condition is obtained from a 3D steady-state flow model of the total catchment area. The 3D model is calibrated using field data previously gathered at the study site. Results indicate that the fully coupled approach with a reduced simulation domain is effective in the simulation of 3D coastal flow dynamics.

Yang, Jie; Graf, Thomas

2013-04-01

5

NASA Astrophysics Data System (ADS)

Data assimilation (DA) has recently received growing interest by the hydrological modeling community due to its capability to merge observations into model prediction. Among the many DA methods available, the Ensemble Kalman Filter (EnKF) and the Particle Filter (PF) are suitable alternatives for applications to detailed physically-based hydrological models. For each assimilation period, both methods use a Monte Carlo approach to approximate the state probability distribution (in terms of mean and covariance matrix) by a finite number of independent model trajectories, also called particles or realizations. The two approaches differ in the way the filtering distribution is evaluated. EnKF implements the classical Kalman filter, optimal only for linear dynamics and Gaussian error statistics. Particle filters, instead, use directly the recursive formula of the sequential Bayesian framework and approximate the posterior probability distributions by means of appropriate weights associated to each realization. We use the Sequential Importance Resampling (SIR) technique, which retains only the most probable particles, in practice the trajectories closest in a statistical sense to the observations, and duplicates them when needed. In contrast to EnKF, particle filters make no assumptions on the form of the prior distribution of the model state, and convergence to the true state is ensured for large enough ensemble size. In this study EnKF and PF have been implemented in a physically based catchment simulator that couples a three-dimensional finite element Richards equation solver with a finite difference diffusion wave approximation based on a digital elevation data for surface water dynamics. We report on the retrieval performance of the two schemes using a three-dimensional tilted v-catchment synthetic test case in which multi-source observations are assimilated (pressure head, soil moisture, and streamflow data). The comparison between the results of the two approaches allows to discuss some of the strengths and weaknesses, both physical and numerical, of EnKF and PF and to learn the implications related to the choice of the statistics used to build the ensemble of realizations.

Putti, M.; Camporese, M.; Pasetto, D.

2010-12-01

6

NASA Astrophysics Data System (ADS)

Limited options exist to measure groundwater processes, particularly at large depths. Coupled time-lapse gravity measurements at the surface and underground are one possibility, but despite recent advances in borehole instruments, no repeat underground gravity measurements of water-mass change have been reported. At the Sanford Laboratory-located at the Homestake Mine in Lead, South Dakota, and site of the proposed Deep Underground Science and Engineering Laboratory (DUSEL)-the U.S. Geological Survey has established a network of 19 surface and 5 underground gravity stations to monitor groundwater-storage change over the projected 20-year existence of the underground laboratory. Continuous pumping is planned to dewater the mine to a depth of 2,500 m; the current pumping regime began in 2007 and current water levels (2011) are at a depth of about 1,700 m. Measurements using a field-portable A-10 absolute gravimeter have been made approximately annually at surface stations since 2007. Underground stations forming a vertical profile along the Ross Shaft on the 300, 800, 2000, 4100, and 4850 levels (numbers indicate approximate depth in feet) were established in 2011, and it is expected all stations will be surveyed annually. To date, surface time-lapse measurements show gravity increases of 50 to 100 nm/s^2 (10 nm/s^2 = 1 microgal) at some stations and equivalent decreases at others, indicating little evidence of water-mass change from pumping. Preliminary modeling, in which the dewatered zone is represented by a series of horizontal prisms that undergo mass change equal to the porosity (assumed 0.005, an average of rock porosity and mined-out voids), indicates that this is the expected result. At pumping rates required to maintain drawdown to a depth of 2,500 m, however, the expected gravity change increases from about 50 nm/s^2 at the surface to 250 nm/s^2 at the 4850 level. Gravity stations in the subsurface are advantageous because they are both closer to the water-mass change from pumping and removed from water-mass change at the surface caused by inter-annual climate variability. Furthermore, the predicted gravity response varies along the vertical profile depending on the distribution of water-mass change in the deep subsurface, indicating the subsurface measurements will be useful for calibrating and validating a groundwater flow model. A tunnel on the 300 level that traverses beneath a forested hillslope and exits at the land surface is a significant future opportunity for monitoring hydrologic processes; concurrent surface and subsurface gravity profiles above and below this hillslope, combined with other measurements, would likely lead to new advances in hillslope-scale hydrology.

Kennedy, J.; Murdoch, L.; Long, A. J.; Koth, K.

2011-12-01

7

Biogeochemical carbon coupling influences global precipitation in geoengineering experiments

NASA Astrophysics Data System (ADS)