Axel Ritter

Untangling complex shallow groundwater dynamics in the floodplain wetlands of a southeastern U.S. coastal river

[1] Understanding the hydrological functioning of tidally influenced floodplain forests is essential for advancing ecosystem protection and restoration goals in impacted systems. However, finding direct relationships between basic hydrological inputs and floodplain hydrology is hindered by complex interactions between surface water, groundwater, and atmospheric fluxes in a variably saturated matrix with heterogeneous soils, vegetation, and topography. Thus, an explanatory method for identifying common trends and causal factors is required. Dynamic factor analysis (DFA), a time series dimension reduction technique, models temporal variation in observed data as linear combinations of common trends, which represent unidentified common factors, and explanatory variables. In this work, DFA was applied to model water table elevation (WTE) in the floodplain of the Loxahatchee River (Florida, USA), where altered watershed hydrology has led to changing hydroperiod and salinity regimes and undesired vegetative changes in the floodplain forest. The technique proved to be a powerful tool for the study of interactions among 29 long‐term, nonstationary hydrological time series (12 WTE series and 17 candidate explanatory variables). Regional groundwater circulation, surface water elevations, and spatially variable net local recharge (cumulative rainfall – cumulative evapotranspiration) were found to be the main factors explaining groundwater profiles. The relative importance of these factors was spatially related to floodplain elevation, distance from the river channel, and distance upstream from the river mouth. The resulting dynamic factor model (DFM) simulated the WTE time series well (overall coefficient of efficiency, Ceff = 0.91) and is useful for assessing management scenarios for ecosystem restoration and predicted sea level rise.

USEFUL SOIL-WATER REPELLENCY INDICES : LINEAR CORRELATIONS

Water repellency (WR) has been classically characterized at fixed (usually oven-dry) soil water content (&thgr;g) in terms of the soil water contact angle (CA), &agr;. However, &agr; has been previously reported to depend upon &thgr;g in a nonlinear fashion, such that WR increases from a wettable state close to saturation (&thgr;g-min) up to a maximum, &agr;max, decreasing afterward either monotonically or rising again to a second local or absolute &agr; maximum nearby the dried soil state. Hence, a CA versus water content (&agr;-&thgr;g) curve may be described in terms of different WR parameters, such as &thgr;g-min, &thgr;g-max, &agr;max, or the integrated area below the &agr;-&thgr;g curve, S. Based on previous &agr;-&thgr;g measurements carried out with the molarity of an ethanol droplet (MED) test, both in mineral and volcanic soils from different world regions, including cultivated and natural forest soils, and textures ranging from clay-loam to sandy, we confirm here the usefulness of the integrated area below the &agr;-&thgr;g curve (S) as a WR describing index for a large variety of &agr;-&thgr;g curve shapes. We found a simple relationship between S and the soil water content at which WR is triggered, &thgr;g-min, such that S = 16.903 &thgr;g-min (R2 = 0.946), which provides an easy method for the rapid characterization of the overall WR degree of soils. S was also linearly correlated with the soil organic matter (SOM) content (R2 = 0.817) for 1 g (100 g)−1 < SOM < 88 g (100 g)−1, such that the best estimate of S was that obtained by combining linearly both &thgr;g-min and the SOM content (R2 = 0.990). Linear correlations were also found between &thgr;g-max, that is, the soil water content at which &agr; is maximum, and S (R2 = 0.834) or the SOM content (R2 = 0.705), and consequently between &thgr;g-max and &thgr;g-min (R2 = 0.830). In addition, both &thgr;g-min and &thgr;g-max were found to depend linearly upon the soil water content at −33 kPa and −1500 kPa, respectively. Finally, a mean soil WR may be defined as the ratio S/&thgr;g-min. We found that the maximum CA, &agr;max, and the mean soil WR S/&thgr;g-min were positively correlated (R2 = 0.780), such that a particular soil with high (low) values of maximum CA is expected to exhibit a high (low) WR degree on average across the whole water regimen from −33 kPa down to oven-dry moisture. Such an estimate of the mean WR index S/&thgr;g-min was further improved if both &agr;max and the SOM content were available (R2 = 0.825).ABBREVIATIONS CA: contact angle; IRDI: Integrative Repellency Dynamic Index; MED: molarity of an ethanol droplet; SOM: soil organic matter; WDPT: water drop penetration time; WR: water repellency.

Hydrological fluxes of a crest laurel subtropical forest in the Garajonay National Park

A study was carried out to quantify the relative importance of the hydrological fluxes involved in crest areas of a laurel subtropical forest ecosystem in the Garajonay National Park (Canary Islands). From a two-year time series of micrometeorological measurements, the forest water inputs and potential evapotranspiration were quantified. Different models were adapted for this type of vegetation to describe physical processes such as potential evapotranspiration, canopy interception losses and the impaction of fog water droplets onto cylindrical elements (needle-like leaves). Conventional precipitation shows seasonality, while impaction models predict that fog precipitation acts as an additional water supply distributed along the year, representing around 20-45% of total yearly rainfall. Due to the prevailing climatic conditions and the stomatal control exhibited by this vegetation, the forest water demand is constrained by reduced yearly evapotranspiration values, which are satisfied mainly by rainfall and additionally by fog interception.