What it is
Mapping where water enters, moves through, pools on, and leaves a site, before you build any earthworks.
The map is a working document, not a static survey. It changes between dry season and wet season, between light rain and storm event. Done well it shows the whole hydrology in a few pages of sketches, contour overlays, and notes.
Every successful restoration begins with this map. Most failed restorations skipped it. P.A. Yeomans built his Keyline system on the principle that you cannot manage what you have not mapped.
Why it works
Water does the heaviest lifting on a site, for or against you. Erosion is water gone wrong. Drought tolerance is water held well. Soil building is water moving slowly through litter and roots. Every restoration outcome traces back to where the water went.
Mapping forces you to walk the land at the right times. You see the gully form during a storm. You watch the seasonal pond appear in March and disappear in June. You notice the spring line on the lower slope that the dry-season survey missed. None of these show up on a topographic map.
Once mapped, the leverage points jump out. A 50 mm rain event delivers 50,000 litres per 1,000 m². If you can see where that water collects, where it accelerates, and where it leaves, you can decide which 200 metres of intervention give you the highest return. Without the map you guess, and most guesses are wrong.
What to map
Catchment boundaries. Where does water entering your site come from. Walk uphill until you cross a ridge. Everything from that ridge down to your boundary is your catchment.
Flow paths. During the next heavy rain, walk the site in waterproofs and trace the surface flow. Mark every channel, every sheet-flow zone, every place water concentrates. Photograph from the same points each visit.
Pooling zones. Where does water sit for hours or days after rain. Note duration (24 h, 72 h, weeks). These are your pond sites, rain garden sites, or wet meadow sites.
Erosion features. Rills (small cuts under 30 cm deep), gullies (over 30 cm), sheet erosion (broad areas of topsoil loss). Each tells you about flow velocity and soil cohesion at that spot.
Springs and seeps. Wet patches that persist through dry season. Often missed on topographic maps. Critical for biodiversity and for siting wells, ponds, and wetland restoration.
Dry channels. Stream lines that flow only in storms. Mark them. Building across an ephemeral drainage is one of the most common and expensive restoration mistakes.
Existing infrastructure. Pipes, drains, culverts, tanks, troughs, irrigation lines. Tile drainage is often invisible from the surface. Ask the previous owner or the local farm advisor.
Soil texture. Sandy patches drain fast. Clay patches hold water and shed it sideways. Texture maps the underground hydrology you cannot see.
Make the map
Use a printed contour base from your national mapping service (Ordnance Survey in UK, USGS in US, IGN in France). Most countries now offer free LiDAR-derived contours at 1 m resolution.
Overlay with hand-drawn sketches in three colours.
Blue. Where water is now (springs, ponds, channels, drains).
Green. Where water lingers after rain (pooling zones, seeps, wet patches).
Red. Where water causes problems (gullies, erosion fans, undercutting).
Number the leverage points. Each point gets a single sentence: "Point 3 -gully head, 1.5 m deep, advancing 20 cm per year." Each one is a candidate for intervention.
Add seasonal notes. Same map, different overlay. February conditions. August conditions. Storm-day notes. The interventions you design have to work across all of these.
Use the map
The map should drive every earthwork decision.
Catchment too large for a single swale. Build a series of swales staggered down the slope, or break the catchment with a check dam at the upper boundary.
Gully head retreating uphill. Drop a rock check dam at the head, then a series down the channel. Stabilises within two seasons.
Pooling zone with 72 h drainage. Candidate for a rain garden or wetland restoration. Plant water-tolerant species. Do not try to drain it.
Sheet flow across compacted ground. Decompact on contour, then build a swale below to catch the slowed flow. See decompaction.
Spring line under degraded forest. Protect the spring. Fence livestock out. Plant native riparian species. Springs are sacred infrastructure in dry country.
Storm channel below settlement. Often the highest priority. Sediment from upslope buries everything downslope in one big event. Build sediment traps and check dams up the catchment, not at the bottom.
The leverage rank is roughly: prevent erosion first, slow water second, store water third, distribute water fourth. Tackle them in that order.
When it goes wrong
Single-season map. You drew the map in summer and missed the winter waterlogged patch. Now your fruit trees are dying in March. Map across at least one full year before major earthworks.
Map without site visits in rain. Pretty contour lines tell you nothing about real flow paths. The single most useful map you ever draw is the one made during a storm.
Acted on the map without testing. A "perfect" swale on a clay perched aquifer ponds water for weeks and kills the berm trees. Always dig test pits in the proposed swale line first. See swales.
Ignored neighbour catchment. Water from upslope land enters your site whether you want it or not. Conversations with uphill neighbours are part of the map. See stakeholder mapping.
Map drawn once, never updated. Hydrology shifts as the project matures. Annual updates catch the new spring that opened after decompaction, the gully that started after a storm, the pond that filled deeper than predicted.