Fortunately, the downhill recipient of any sheet flow from this property would be the BLM. I'm assumingthat such a neighbor isn't going to object to a swale, but I have no way of asking for any such permission. I do understand that water in Wyoming is, legally, the property of the state; but everything that I can find implies that water catchment from an owned property is still considered a human right, up to an annual limit. That limit, iirc, wasn't very high at all (around 850 gallons of storage, there abouts) which certainly puts a hard limit on the size of any cistern I were to install, but I was thinking about 350 gallons anyway; at least in the beginning, as this is just going to be a camping site for many years. What I can't determine is, does the law count what is stored in the dirt? So far, it looks like that is a no, since natural sheet flows are expected to either soak in or evaporate most of the time anyway; but I know I'm in dangerous territory. Wars have (literally) been fought over less.
I could start out with a small swale, that doesn't interrupt the majority of the sheet flow; or even just a rock ring/dam. Mostly, I want the cistern & catchment to be invisible to the uninitiated eye; but capture & hold enough water that it could be useful as a hunters' base camp. I could pay for the annual taxes simply by leasing out the site as a private campsite during hunting season. As Paul has mentioned regarding his own experiences, the Department of Making You Sad usually responds to complaints, and otherwise doesn't have the resources to hunt down victims that don't pop their heads up.
So if my cistern is 350 gallons, and my goal is to have it refill every month if necessary; the lowest average monthly rainfall seems to be just about half an inch in August. I've been told that one square foot of (impervious) catchment will yield one-half gallon of water per inch of rainfall. So to refill my cistern from empty during an average August, I'd need at least 1400 square feet of watershed. That seems like it should be trivial on a 40 acre property. A swale or rock dam 15 feet across, 100 feet from the highest point of the property and directly in line with sheet flow should be able to get that much done. Double either metric to compensate for soakage or other losses, and we're good.
I absolutely don't count on the swale as a needed feature, however; as the cistern would still capture a respectable amount of water without such an earthwork. I figured that it would be a value-added kind of thing for the property given time, as it might be able to support more trees; should i decide to either move there myself or sell the property.
Subject: Raw land in Wyoming
I would strongly urge you to double check the water rights for the property (you want to know how old they are among other things) And really do some reading up on water laws in Wyoming. In some places it is illegal to build swales etc that will prevent water from flowing from your property onto the property of a person below you in the watershed with older water rights...
I post this here because I want some opinions of people who live a lot closer than I do. This property borders BLM land, so there's the advantage that neighbors are unlikely to bother me; also, direct access to public land. It's a highland area, and only drops 20 feet or so from the NW corner to the SE corner; so very flat, but a couple of swales might work wonders. Rain is about 10 inches per year, so definitely a dry land. It's nearly 2 hours from the closest Wal-mart, which is in Rawlins; so this is quite a bit past the middle of nowhere. I know that Paul recommends more elevation change, and more rainfall; but the property is only $500 per acre and borders another 640 acre square of BLM land. I was thinking, build a swale & bury a cistern. French drain (inside swale), fabric wrapped as a large particle filter, drains directly into the cistern. Standard shallow well manual pump nearby for water access. Hang a sign on it that says "not potable" or some such, just in case hikers cut through; or simply cap the pipe and take the pump while I'm not there. Come back in a year for a camping week, plant some tree seeds into that swale while I'm there. Next year or so, come back and build the other swale. I understand that it takes about 7 years for the swale to max out what the local water storage can be. Eventually (after I retire, or my employer releases me to "pursue other opportunities") I could move out there and build an off-grid home; otherwise I could simply sell it and reasonably expect that the swales would have increased the land value anyway.