Personal computer as a tool for sustainable development For the purpose at hand there is another restriction on a mathematical model to be useful, it should be easily implemented on a PC and the use of the software based on it must not require any specialized knowledge. This requirement is becoming less restrictive as the power of PCs continuously increases. Closely related to the idea of a mathematical model is the concept of computer simulations. Once we have a mathematical model we can convert it into a set of instructions that can be repeated by a computer as often as we wish. This way we can try different scenarios see their consequences and then choose the most desirable option. Thus a mathematical model of a pond that takes into account the fluctuations in the rainfall, the runoff generated, evaporation losses, and the percolation losses allows us to predict the amount of water that will be available in a given pond during the course of the year without actually having to build the pond! This gives community a tool in helping to decide whether to build a pond or not, and if to build then the model informs it about the size of the pond and the catchment area. SimTanka - An Example of Modeling Water Resources To make the ideas of the previous section concrete I will consider a simple and useful example of a mathematical model, and show that how it can be used in meeting our objective of sustainable development. The example I will consider is a mathematical model of rainwater harvesting system with covered storage tanks; the model has been implemented on a PC via software called SimTanka . SimTanka models the performance of a rainwater harvesting systems with covered storage tank. Such systems have been used for more than a millennium in various parts of the world. In western Rajasthan, where their use was very wide spread, they are called Tanka. By performance of a Tanka I mean the ability of a Tanka to meet a given demand. So what SimTanka aims to do is to determine to what extent a given Tanka would meet the demand made on it, and with what degree of reliability. In order to see how SimTanka does this let us follow the sequence of events that takes place when user uses it. As Fig. 1 shows, the user is asked to enter the location where the Tanka is situated or where it is being planed to build. In the above illustrative e.g. the location of Tanka is assumed to be in Bikaner district of Rajasthan. The rainfall data is an essential information for simulating the performance of the Tanka. As we can see from the representative data of Fig. 2, there are extreme fluctuations in the rainfall. This is a general characteristic of the rainfall pattern in arid and semi-arid regions. These fluctuations in rainfall will obviously reflected in the amount of water collected and stored in the Tanka. If we want to predict the future performance of the Tanka, we cannot use a mean value, say the average monthly rainfalls. That would be misleading. Nor do we have any mathematical model that can predict the future rainfall; the best that we can do is to use the past rainfall record as an indicator of the fluctuations in the future rainfall. It is for this reason that SimTanka needs actual monthly rainfall data for at least last fifteen years, and not just the average monthly rainfall. I will not go into the details of the manner in which SimTanka uses past rainfall data to simulate future probabilities , for our purpose, it is sufficient to know that SimTanka does take into account the various draughts that may occur in the future. Also notice that the user is asked to select the degree of reliability, this is the reliability against the fluctuating rainfall. For e.g. if the user chooses, as in Figure 1, "Extremely Reliable (95%)" then SimTanka will find the minimum catchment area and the smallest possible tank size than can meet he water demands 95% of time, in spite of the fluctuations in the rainfall. Higher reliability, as we will see, comes at the cost of larger catchment area and or bigger tank size.  Figure 1. Opening screen shot of SimTanka  Figure 2. Time-Series of Annual Rainfall Record for Chaksu (Raj.), India. |