The following operational definition enables different observers to classify the functionality of a water source in a consistent way: “A functional water source is one with water available at the time of inspection, such that an enumerator could fill a 20-L container within 10 minutes.” This is not necessarily superior to other definitions, but it can be feasibly measured in the field with adequate reproducibility using readily available equipment and is sufficiently specific and inclusive to enable the classification of most water sources under most circumstances.