Groundwater Level Dynamics Research Articles

Changes in groundwater level dynamics after low-impact forest harvesting in steep, small watersheds

Summary Groundwater in steep watersheds has complicated dynamics because hydrological processes are strongly affected by topography. To clarify the influence of harvesting on groundwater levels in hillslope forests, the pre- and post-harvest relationships between rainfall and groundwater levels were examined focusing on the frequency and elevation of observed groundwater levels. The dynamics of groundwater levels changed in two ways. First, the watering-up frequency, defined as the ratio of non-zero groundwater frequency to total observations increased in the entire watershed, except in wells in which the pre-harvest watering-up frequency was sufficiently high as to prevent further increase. Second, increased watering-up ratios, defined as the ratio of groundwater level to soil depth in each class of groundwater level sorted in descending order similar to the discharge–duration curve, were detected in the higher class of groundwater level for wells located in the upper riparian zone. These increased watering-up ratios were interpreted as the expansion of saturated areas observed mainly during heavy rainfall events. The reduced interception rate after harvesting resulted in increased soil moisture, which led to increases in watering-up frequencies and ratios. In addition, changes in the physical properties of surface soil may also lead to increased maximum groundwater levels because of decreasing hydraulic conductivity for lateral flow in the upper soil layer. This study suggests that the groundwater regime in hillslope forests differs spatially according to the topographic conditions, and the spatiality changes depending on the growth stage of vegetation.

Emissions of N2O from fertilized and grazed grassland on organic soil in relation to groundwater level

Intensively managed grasslands on organic soils are a major source of nitrous oxide (N2O) emissions. The Intergovernmental Panel on Climate Change (IPCC) therefore has set the default emission factor at 8 kg N–N2O ha−1 year−1 for cultivation and management of organic soils. Also, the Dutch national reporting methodology for greenhouse gases uses a relatively high calculated emission factor of 4.7 kg N–N2O ha−1 year−1. In addition to cultivation, the IPCC methodology and the Dutch national methodology account for N2O emissions from N inputs through fertilizer applications and animal urine and faeces deposition to estimate annual N2O emissions from cultivated and managed organic soils. However, neither approach accounts for other soil parameters that might control N2O emissions such as groundwater level. In this paper we report on the relations between N2O emissions, N inputs and groundwater level dynamics for a fertilized and grazed grassland on drained peat soil. We measured N2O emissions from fields with different target groundwater levels of 40 cm (‘wet’) and 55 cm (‘dry’) below soil surface in the years 1992, 1993, 2002, 2006 and 2007. Average emissions equalled 29.5 kg N2O–N ha−1 year−1 and 11.6 kg N–N2O ha−1 year−1 for the dry and wet conditions, respectively. Especially under dry conditions, measured N2O emissions exceeded current official estimates using the IPCC methodology and the Dutch national reporting methodology. The N2O–N emissions equalled 8.2 and 3.2% of the total N inputs through fertilizers, manure and cattle droppings for the dry and wet field, respectively and were strongly related to average groundwater level (R 2 = 0.74). We argue that this relation should be explored for other sites and could be used to derive accurate emission data for fertilized and grazed grasslands on organic soils.

The dynamics of cultivation and floods in arable lands of Central Argentina

Abstract. Although floods in watersheds have been associated with land-use change since ancient times, the dynamics of flooding is still incompletely understood. In this paper we explored the relations between rainfall, groundwater level, and cultivation to explain the dynamics of floods in the extremely flat and valuable arable lands of the Quinto river watershed, in central Argentina. The analysis involved an area of 12.4 million hectare during a 26-year period (1978–2003), which comprised two extensive flooding episodes in 1983–1988 and 1996–2003. Supported by information from surveys as well as field and remote sensing measurements, we explored the correlation among precipitation, groundwater levels, flooded area and land use. Flood extension was associated to the dynamics of groundwater level. While no correlation with rainfall was recorded in lowlands, a significant correlation (P<0.01) between groundwater and rainfall in highlands was found when estimations comprise a time lag of one year. Correlations between groundwater level and flood extension were positive in all cases, but while highly significant relations (P<0.01) were found in highlands, non significant relations (P>0.05) predominate in lowlands. Our analysis supports the existence of a cyclic mechanism driven by the reciprocal influence between cultivation and groundwater in highlands. This cycle would involve the following stages: (a) cultivation boosts the elevation of groundwater levels through decreased evapotranspiration; (b) as groundwater level rises, floods spread causing a decline of land cultivation; (c) flooding propitiates higher evapotranspiration favouring its own retraction; (d) cultivation expands again following the retreat of floods. Thus, cultivation would trigger a destabilizing feedback self affecting future cultivation in the highlands. It is unlikely that such sequence can work in lowlands. The results suggest that rather than responding directly and solely to the same mechanism, floods in lowlands may be the combined result of various factors like local rainfall, groundwater level fluctuations, surface and subsurface lateral flow, and water-body interlinking. Although the hypothetical mechanisms proposed here require additional understanding efforts, they suggest a promising avenue of environmental management in which cultivation could be steered in the region to smooth the undesirable impacts of floods.

Dynamic analysis of groundwater discharge and partial-area contribution to Pukemanga Stream, New Zealand

Abstract. The proportion and origin of groundwater contribution to streamflow from agricultural catchments is relevant to estimation of the effects of nitrate leached from the soil on the quality of surface waters. This study addresses the partitioning of streamflow contributions from near-surface runoff and from groundwater, each with different contributing land area, on a steep pastoral hillslope in a humid climate. The 3 ha headwater catchment of the perennial Pukemanga Stream, in the North Island of New Zealand, was instrumented for continuous observation of climatic data, streamflow and groundwater level. The dynamics of groundwater levels and groundwater contribution to streamflow were analysed by means of a one-parameter, eigenvalue-eigenfunction description of a 1-D aquifer model. Model results for seven years of daily data predict that 36–44% of the topographical catchment contributes groundwater to the stream. The remaining groundwater generated within the catchment contributes to streamflow outside the catchment. Groundwater was calculated to be 58–83% of observed annual streamflow from the topographical catchment. When the smaller groundwater catchment is taken into account, the groundwater contribution to streamflow is 78–93% on a unit area basis. Concurrent hourly data for streamflow and groundwater levels at two sites indicate the dynamic behaviour of a local groundwater system. Groundwater flow dynamics that support the perennial nature of this headwater stream are consistent with the size of the groundwater body, porosity of the subsurface material, and hydraulic conductivity derived from partitioning of streamflow contributions.

The Basin of Mexico aquifer system: regional groundwater level dynamics and database development

The aquifer system of the Basin of Mexico is the main source of water supply to the Mexico City Metropolitan Zone. Management of the Basin’s water resources requires improved understanding of regional groundwater flow patterns, for which large amounts of data are required. The current study analyses the regional dynamics of the potentiometric groundwater level using a new database called the Basin of Mexico Hydrogeological Database (BMHDB). To foster the development of a regional view of the aquifer system, data on climatological, borehole and runoff variables are part of the BMHDB. The structure and development of the BMHDB are briefly explained and then the database is used to analyze the consequences of groundwater extraction on the aquifer’s confinement conditions using lithology data. The regional analysis shows that the largest drawdown rates are located north of Mexico City, in Ecatepec (a region that has not yet received attention in hydrogeological studies), due to wells that were drilled as a temporary solution to Mexico City’s water-supply problem. It is evident that the aquifer has changed from a confined to an unconfined condition in some areas, a factor that is responsible for the large subsidence rates (40 cm/year) in some regions.

Improving model structure and reducing parameter uncertainty in conceptual water balance models through the use of auxiliary data

The use of uncertainty analysis is gaining considerable attention in catchment hydrological modeling. In particular, the choice of appropriate model structure, identifiability of parameter values, and the reduction of model predictive uncertainty are deemed as essential elements of hydrological modeling. The chosen model structure must be parsimonious, and the parameters used must either be derivable from field‐measured data or inferred unambiguously from analysis of catchment response data. In this paper, a long‐term water balance model for the Susannah Brook catchment in Western Australia has been pursued using the “downward approach,” which is a systematic approach to determine the model with the minimum level of complexity, with parameter values that, in theory, are derivable from existing physiographic data relating to the catchment. Through analysis of rainfall‐runoff response at three different timescales and exploring the climate, soil, and vegetation controls on the water balance response at these timescales, an initial model structure was formulated, and a priori model parameter values were estimated. Further investigation with the use of auxiliary data such as deuterium concentration in the streamflow exposed inadequacies in the chosen model structure. Two more model structures were then proposed and investigated through formulating alternative hypotheses regarding the underlying causes of observed variability, including those suggested by observed deuterium composition in the streamflows and observed groundwater level dynamics. Along the way, the resulting models were systematically evaluated in three dimensions: ability to reproduce observations, predictive uncertainty, and physical realism. The final model, which included an efficient but detailed representation of unsaturated zone time delay, was found to be superior on all three counts, i.e., improved performance, reduced predictive uncertainty, and improved physical realism. These improvements can be directly attributed to the use of the auxiliary data (deuterium composition and groundwater level dynamics), which identified weaknesses in previous model structures, and the multiple wetting front model of water movement in the unsaturated zone, which captured more effectively the time delay in the unsaturated zone.

3D geophysical processing, visualisation and interpretation of rice irrigation impacts

The National Centre for Groundwater Management at the University of Technology, Sydney has completed a 3-stage research project at a field site near Griffith NSW, which included an examination of the dynamics of groundwater levels and salinisation adjacent to an irrigated rice paddock. The site had 22 piezometers and was instrumented with 16 dataloggers from October 1999 to September 2001. Ten resistivity imaging and EM34 surveys were conducted during the period October 1998 to June 2001 ? before, during and after irrigation. Some EM31 was also done. This paper will concentrate on the collation and qualitative interpretation of the resistivity imaging data. Each imaging survey comprised 15 lines (during winter) or 10 lines (during irrigation) each of about 200 metres length. The lines were generally 10 metres apart (sometimes 5 m). The results from each survey have been assembled into a movie (using SURFER and SLICER software) which shows the 3-D heterogeneity in resistivity across the site. A couple of movies have been produced showing the variation with time along representative sections. Snapshots of the differential apparent resistivity between surveys before and after irrigation show definite incremental salinisation in the fallow paddock adjacent to the irrigated rice paddock.