Sustainable groundwater resources exploration and management in a complex geological setting as part of a humanitarian project (Mahafaly Plateau,Madagascar)

Southwestern Madagascar is a semi-arid region and a hot-spot of global change. On the Mahafaly plateau, people live with quasi-permanent water stress and groundwater, the only available resource, is difficult to exploit due to a complex hydrogeological environment. A methodology (suitable for humanitarian projects; <?40 k€) was developed in four phases to assess the sustainable exploitation of the water resource: (A) regional scale exploration, (B) village scale exploration, (C) drilling campaign, and (D) hydro-climatic monitoring. This integrated hydrogeophysical approach involves geophysical measurements (262 TEM-fast soundings, 2588 Slingram measurements, 35 electrical soundings), hydrochemical analyses (112 samples), and a piezometric survey (127 measurements). Two groundwater resources were identified, one deep (below 150 m) and one shallow (<?20 m). Hydrochemical results highlighted the vulnerability of both resources: anthropic contamination for the shallower and seawater intrusion for the deeper. Therefore, subsequent geophysical surveys supported the siting of six boreholes and three wells in the shallow aquifer. This methodological approach was successful in this complex geological setting and requires testing at other sites in and outside Madagascar. The study demonstrates that geophysical results should be used in addition to drilling campaigns and to help monitor the water resource. In fact, to prevent over-exploitation, piezometric and meteorological sensors were installed to monitor the water resource. This unique hydro-climatic observatory may help (1) non-governmental organization and local institutions prevent future water shortages and (2) scientists to understand better how global change will affect this region of the world.     

Broad classification of New Zealand inlets with emphasis on residence times

Thirty‐two New Zealand coastal inlets are tentatively classified into seven groups with probable similar circulation patterns, based on ratios of their physical parameters: volume, tidal compartment, entrance width, surface area, length, and average width. Eighteen of these have a predominant tidal flow, shown from their small ratio (ß < 4) of tidal compartment to total volume, and the cross‐sectional areas of their entrances are controlled by the tidal flow: Moutere, Waimea, Aotea, Whanganui, Avon‐Heathcote, Tauranga, Parengarenga, Porirua‐Pauatahanui, Kawhia, Nelson, Rangaunu, Raglan, Whangarei, Bluff, Otago, Hokianga, Manukau, and Whangaruru. The other fourteen range from long narrow sounds with probable strong vertical circulation (e.g., Pelorus Sound) to large bays with strong mean horizontal circulations (e.g., Tasman and Hawke Bays).

Examples of residence time calculated in inlets with ß > 4 are given for Tasman Bay, a bay with a mean horizontal circulation, which has a residence time of 1–3 months, and Pelorus Sound, an inlet with a vertically complex circulation, which has a residence time of about 20 d.     

Circulation in Tasman Bay

Direct current measurements at four locations in Tasman Bay and numerical model results are used to analyse the mean flow in Tasman Bay. The mean circulation conforms to that previously found from drift card experiments: a clockwise circulation in Golden Bay, and an anti‐clockwise flow in Tasman Bay, with a return south‐westerly flow on the coast near Nelson. Typical mean speeds are 0.02–0.05 m.s^‐1. The circular flow appears asymmetrical in both bays, with a stronger outflow along Farewell Spit in Golden Bay and near D'Urville Island in Tasman Bay.

An analytical tidal solution is used to exhibit the influence of Cook Strait in producing smaller tidal amplitudes in eastern Tasman Bay. Tidal speeds of 0.15–0.30 nus^‐1 are typical, with tidal ellipses having degenerated into north‐east, south‐west lines.     

Oceanic circulation in the head of the Hikurangi trench,east coast,New Zealand

The geostrophic flow and direct current measurements in the inflow of the Southland Current into the Hikurangi Trench east of New Zealand confirm the geostrophic circulation pattern previously found. The northwards inflow of the Southland Current at the latitude of the Chatham Rise (44°S) is estimated at 1.7 × 10⁶ m³.s^‐1, which compares favourably with that of 1.2 × 10⁶ m³.s^‐1 calculated previously off Kaikoura.     

Low cloud boundaries coincident with oceanic convergences

Low stratus cloud was present over the cool water on the southern and western sides of the Subtropical Convergence east of New Zealand during two airborne infrared radiation thermometer surveys made on 2 April and 17–18 November 1969. The alignment of the northern boundary of such cloud with the Subtropical Convergence, and of cloud boundaries elsewhere with other areas having large horizontal temperature gradients, demonstrates that often such cloud boundaries can be useful indicators of surface temperature discontinuities.     

Sea level oscillations in Wellington Harbour

Periods of oscillations of sea level in Wellington Harbour, New Zealand (41° 17'S, 174° 52’ E), are calculated by spectral analysis of the residual elevations observed in tide gauge records. These periods are compared with those computed by numerically integrating a one‐dimensional linear momentum equation and the continuity equation. The two main oscillations are the first harmonics along and across the harbour with periods of about 27 and 22 min respectively — the second harmonics were also observed. The quarter wavelength oscillation with forcing at the mouth which was excited by the 1960 Chilean Tsunami has a period of about 160 min.