The amount of instantaneous waterflow of a river system depends on the rains, which is dependant on the season. The instantaneous waterflow varies from day to day with a minimum therof, located usually at the end of the dry season if it is marked. The concept of average flow has no interest in powerplants "along the waterstream", however, it does allow to better estimate the potential energyoutput of an infrastructure if an accumulation is envisaged. Low water flow, ie the minimum flow of the river during 24h states the minimal poweroutput potential of an installation. If the hydrological observations (measures of the flow of the river) are done for several years, it is possible to know the average minimum waterflow attained annually, or it is possible to observe it every 5 years, or -even more rare-, every 10 years. Indeed, the severity of the drought is variable depending on the year. A flow measure during 365 days can not indicate whether the observed minimum is an exceptional speed (either low or high) or rather an average minimum.
The hydrological data may be essential for the design of the proposed small hydroelectric plant. A lack of flow and thus availability of water will lead to disillusionment when the installation is working due to the large gap between the expected power output and true available power. There is of course no need to seek accurate hydrological data if the power output of the proposed installation is well below the maximum power of the site chosen for the project. Given that the turbine is to be placed near the river, it is highly desirable to know the variations of water level, to avoid seeing water invading the facilities during floods.
===Constraints for exploitation===
# A small hydroelectric powerplant is very expensive to install, around 1500 € to 3000 € per installed kW; operating costs are however quite low. The material is generally robust and lasts a lifetime if regular maintenance is exerciced for several decades. If we sign up for the perspective of a sustainable development, we should consider that in terms of the economic life-expectancy of the facility, the replacement of the facility will be incorporated by the users of the network agreeing to pay for the supplied power. The billing of the consumption allows to build up the needed capital for the maintenance and future replacement; it avoids abusive consumptions. About the pricing, it must be designed in function of the objectives of the powerplant project. About the pricing, we suggest progressive pricing (the more one consumes, the more the price per kW is increased, obviously within certain limits) which reduces the risk of waste, and sectorial pricing (collective or community consumptions are are charged at more favorable rates). See http://www.ciele.org/filieres/hydraulique.htm
# Operating costs are low if the facility is regularly maintained: the damaged parts are replaced quickly (stock of spare parts), abnormal wear is investigated to identify the causes. Losses on the network are eliminated without delay.
# Who says maintenance, says manager and personnel trained for the task at hand.
# Control of consumption and illegal connections are punished. A too great consumption (overloading the network) will lead to a voltage drop that can cause damage to connected equipment, especially engines. The client is asked to sign a service contract which stipulates his rights and obligations.
# Public information campaign on things to avoid and prevention: The area of the water intake in the river is to be avoided by the other users of the river (risk of getting caught up in the waterflow and risk of drowning); intake channel may be used for other purposes (laundry, irrigation), the manager informs the public to incidents such as blocked grates, overflow, etc ... The practice of bathing is regulated or prohibited in the absence of a basin with restraints to prevent the risk of drowning or sudden spills of large volumes of water. Information on the dangers of electrocution will be presented at both the facility and at the home.
# In the spirit of sustainable development, the facility must take into account other water uses , both existing or planned, at the time of the drafting of the powerplant project. The sharing of the water must be done fairly; ie on the basis of criteria that all stakeholders comply to.
# Estimates of budget, deadline for implementation: From an average unit cost of 2000 €/kW installed, it is easy to estimate the investment budget for 10, 100 or 1000kW, as being € 20,000 to € 2,000,000. On the other hand, pico-powerplants of 300W are sold in Vietnam for U.S.D. $20 and are very popular. It is likely that the complexity of a facility of more than 100kW extends the delay of realisation because of forgoing feasibility studies, which probably increases the quality and reliability of the project. Compact installation kits are sold in commerce, requiring only a minimum of know-how for the installation. This equipment series offers the advantage of being available quickly and to be well calibrated for operating conditions. Unless you have control of all parameters (location, project funding, expertise for the construction of infrastructure), an installation project requires several years between the first sketches on paper and the effective operation of the distribution network.
# Small powerplants and local development: Many places on earth are not yet equipped with electric network due to lack of interest (economic or political) to install utilities. The low population density is one of the main reasons for this lack. A micro-powerplant offers a solution for decentralised development where the local undertaker or local authorities can act autonomously. A well designed and well maintained installation offers a very low return price, within the reach of small and medium enterprises. The social impact of these small facilities is considerable in the fight against poverty. See http://www.tve.org/ho/doc.cfm?aid=1636&lang=English
The Chinese authorities have helped a great amount of areas removed or isolated from major electricity distribution networks in acquiring small powerplants. Similar projects have been developed, especially in Kenya.
These measures allow the local population to develop activities that meet their expectations and to avoid, for example, migration to the already congested suburbs of major cities. These populations can emerge from the isolation, communicate with the rest of the world and participate in the evolution.