Water, water everywhere...

Just to get the designer wheels in your head started spinning, Peak Energy posted a good link to a story about a successful ground heat pump.

I have a little bit more to say about basic requirements as discussed in the previous post. Looking at Objective 2.1 (“The community will provide indoor plumbing adequate for four people.”), I’m starting to think that the driving goal (Goal 2) should be changed so that we can use it to address all the water management needs. This type of modification is common as Systems Engineering solutions are developed and I hope it shows you how fluid the design process is.

So, instead of Goal 2 reading “The community will have indoor plumbing”, how about “The community will provide adequate water supplies.” We’ll leave Objective 2.1 as it is, but add Objective 2.2: “The community will provide enough water to sustain four people.” Now that we fixed our goals and objectives, let’s define some requirements.

Taking some liberties with the data, the average human needs approximately 5 liters per day (lpd) for drinking, 45 lpd for hygiene and bathing, 20 lpd for cooking activities, and 3500 lpd for food production according to the Pacific Institute. (Very good link by the way.) Bear in mind these numbers reflect only the water “at the tap” – they take into account moderate efficiency improvements over the average American’s lifestyle and say nothing about recycling or help from local rainfall.

I will put these numbers into two categories: personal and agricultural, with respective values of 70 lpd and 3500 lpd. For the four-person community, this comes to 280 lpd for personal water and 14,000 lpd for agricultural water. This leads naturally to two requirements – “The community shall provide no less than 280 lpd of indoor water.” (Requirement 2.1.1) and “The community shall provide no less than 14,000 lpd of water for food production.” (Requirement 2.2.1)

Notice two things about these requirements: 1) They are wild-ass estimates and will surely be modified as your design develops and 2) they are intertwined with each other and their parent objectives. I could just as easily put Req 2.1.1 under Objective 2.2. Assigning requirements often comes down to intuition or preference, but remember that in the end every objective (for any project) must have at least one requirement. This ensures that every objective is met.

In future posts I will talk about planning for changes in your local water allocation due to global warming effects, aquifer depletion, or upstream pollution.