Where Monsoon Water Goes on Your Parcel: Reading an Elevation Model

The National Weather Service runs the monsoon season from 15 June to 30 September. The 2026 season is over. If water came across your parcel in a way you did not expect — through the yard, across the drive, around rather than through the culvert — the evidence is lying on the ground right now, and it will be gone by spring.

This article is about reading that evidence from the air, what the FEMA map does and does not tell you in unincorporated Cochise County, and what to do with the result. We produce the model. We do not design drainage, and we will say where that line is.

How water moves in this valley

The Sulphur Springs Valley fills rather than drains. The USGS describes the Willcox basin as "an area of interior drainage in the northern part of Sulphur Springs Valley" where, during and after the faulting that formed it, "large quantities of alluvium were deposited in the closed basin." Water that falls on the Dragoons, the Chiricahuas, the Dos Cabezas or the Winchesters comes down a canyon, hits the valley floor, and spreads. Three patterns, from mountain front toward playa:

• Washes. Defined channels with banks. Easy to see where the channel is deep; hard to see where it shallows, braids, and becomes the next pattern.
• Alluvial fans. Where a wash leaves its canyon it drops sediment in a fan and splits into distributary channels that shift from storm to storm. The county's floodplain regulations note that some Zone AO has been designated "in areas with high flood velocities such as alluvial fans and washes." Last year's channel is not necessarily this year's.
• Sheet flow. The Arizona Department of Water Resources' standard calls it "a loosely defined term, as it is used in Arizona," with one primary characteristic: a significant part of the floodwater is not conveyed in a single, well-defined channel. Cochise County defines a sheet flow area as one with depths of half a foot or more in the base flood and no clearly defined channel.

Sheet flow is the one that catches people. A parcel walked in May shows grass, creosote and ground that looks level. The same parcel in a September storm has four inches of moving water across the whole east half, because the parcel is a very wide, very shallow channel.

What an elevation model shows that a site walk does not

A mapping flight produces an orthomosaic — a measurable aerial photograph at a few centimetres per pixel — and a surface model, a grid of elevations across the parcel. For drainage, the second is the point and the first is the proof.

From the surface model, standard GIS hydrology tools compute, for every cell, which way water leaves it and how much ground drains through it. Rendered over the orthomosaic, that gives you flow paths, including the shallow ones with no visible bank that a walk misses; low points, and how deep they fill before they spill; where water arrives from the neighbour and where it leaves, which matters under county rules that limit how much a development may raise flood levels; and how far above the nearest flow line a proposed pad actually sits. Not a FEMA base flood elevation. A number you can reason with.

Then the orthomosaic, flown in October, shows whether the model is right. Scour lines, debris lines, flattened grass, fresh sediment fanned out below a culvert, the dark stain of a recently dry pond — all visible from 200 ft, most of it invisible from the gate. Where prediction and evidence agree, trust the model. Where they disagree, you have learned something about the parcel.

What the FEMA map does and does not tell you

FEMA's Flood Map Service Center is "the official public source for flood hazard information" for the National Flood Insurance Program, and the Flood Insurance Rate Map it serves is what a lender and the county look at first.

The map is a regulatory product, not a drainage study. It delineates the 1% annual chance flood for watercourses that were studied. The county's regulations acknowledge the gap: the FEMA maps "define the minimum area of applicability" of the ordinance, the regulatory floodplain also includes sheet flow areas and watercourses carrying 500 cubic feet per second or more whether mapped or not, and the Floodplain Administrator "may require the land owner to establish the regulatory floodplain and floodway limits" by study where none exists. Outside the shaded area does not mean outside a floodplain.

The map is being revised. FEMA issued preliminary FIRM panels for Cochise County, Benson, Tombstone and Willcox dated 11 October 2024, and on 29 January 2026 published proposed flood hazard determinations covering Benson, Bisbee, Douglas, Tombstone and the unincorporated county, with comments due by 9 March 2026. Check the Map Service Center and the county, not a printout from a closing.

The map was not drawn for your pad. A FIRM panel covers square miles at a scale where a 40-acre parcel is a thumbnail. "Will water cross the spot where I want the house" is below its resolution. That is what the elevation model is for.

Why October

Because the evidence is fresh. Debris lines, scour and sediment deposits are at their most legible for a few weeks after 30 September, before wind, cattle and the first winter rain blur them. In our experience it is also the most reliable flying month here — calm mornings, no afternoon build-up, dry air.

The trade-off is grass. Post-monsoon is when the grass is tallest, which is when a surface model is furthest from the ground. On most valley-floor parcels that is a few centimetres and does not matter. Where it does — a pad site in a mesquite thicket — the honest plan is an October flight for the orthomosaic and flow evidence, and either bare-earth classification (+$800) or a second short flight in February for clean ground. We will say which your parcel needs rather than sell you both.

And if the water already came in, the same October flight, with the roof included, is the documentation for the claim. Our article on documenting monsoon roof damage covers that side.

What this costs, and what to do with it

The minimum mapping call-out on our rate card is $750 for up to 10 acres, orthomosaic and surface model; ground control is +$450 and you want it if depth or pad height is the question; 1 ft contours are +$450 if an engineer or the county will want a drawing. There is no travel charge in the Sulphur Springs Valley, and most of the county is Class G airspace, so in October we can usually fly within days.

Then take the result to the county and, if you are building, to an engineer. The Floodplain Administrator for unincorporated Cochise County is the County Engineer, and the regulations require a Floodplain Use Permit before any construction or land disturbance in a regulatory floodplain or FEMA special flood hazard area; applications go through the county's online permit portal, linked below. Where more than an acre is disturbed, the regulations call for a drainage analysis sealed by an Arizona Registered Professional Civil Engineer, and our point cloud is a legitimate input to that work.

Whether the result means moving the pad, cutting a diversion swale, sizing a culvert or doing nothing is a judgement for the engineer and the county, and we will refer you to both rather than guess. The topographic mapping page has the full deliverable list.

If you want the parcel flown while the evidence is still on the ground, send the parcel number and what you saw this summer through the contact form.