Getting the right help
A hydrologist works with surface water: rainfall, runoff, catchments, rivers, flooding and drainage — everything that happens on top of the ground. A hydrogeologist works with groundwater: aquifers, bores, water table behaviour, groundwater licensing and contamination transport — everything below it. If your question involves a flood level, a drainage design, a culvert, a creek or a stormwater condition on a development approval, you want a hydrologist. If it involves a bore, a groundwater allocation, dewatering, a water table level or effluent disposal, you want a hydrogeologist.
What a hydrologist does on a real project
Quantifies how water moves across and off the land. How much rainfall becomes runoff, where it goes, how deep and how fast it flows in an extreme event, and what infrastructure is needed to manage it safely.
On a development that usually resolves into three deliverables. Establishing flood levels, which means a design rainfall, a catchment model and a hydraulic model, calibrated where there is anything to calibrate against. Showing whether the proposal changes flood behaviour — whether it raises levels on neighbouring land, obstructs a floodway or removes flood storage. And designing the drainage that follows: overland flow paths, pipe and culvert capacity, detention or retention storage, and the erosion and sediment control that keeps the design working during construction rather than only after it.
On infrastructure the questions change but the discipline does not: flood immunity for a road, rail corridor or critical asset; the hydraulic design of a culvert, levee or basin; the realignment of a watercourse around something that has to be built where the creek currently is.
Where the two genuinely overlap
Groundwater–surface water interaction is the honest overlap, and on the Swan Coastal Plain it is not a footnote. Where soils are sandy and the water table is shallow, drainage design is set by separation to the maximum groundwater level rather than by pipe capacity — the City of Swan, for example, requires subsoil drains wherever the maximum groundwater level sits less than 1.2 metres below proposed lot levels, and requires that level to be established from on-site monitoring rather than assumed. That is a hydrogeological input feeding a hydrological design, and a project that treats them as separate scopes usually discovers the connection late.
The same happens in reverse on baseflow-dependent catchments, on mine dewatering discharging to a surface water system, and wherever acid sulfate soils sit within reach of an excavation or a lowered water table.
The questions that sort themselves
Surface water, so a hydrologist: what is the 1% AEP flood level on my land; will this development flood my neighbour; how big does this culvert need to be; the council has asked for a stormwater management plan; the flood mapping does not cover my site; will this dam affect downstream flow; the road washes out every winter.
Groundwater, so a hydrogeologist: how deep will I have to drill; can I get a licence to take groundwater; what is the maximum groundwater level on this lot; is my bore going saline; can this site take an on-site effluent disposal system; how much dewatering will this excavation need.
And a third category worth naming, because it is neither: bushfire attack level assessment, flora and fauna survey, contaminated site investigation, geotechnical testing and land surveying are separate disciplines that often appear on the same approval checklist. A surface water practice will tell you that rather than quoting for it.