Finding the Balance Between Evaporation and Rainwater Harvesting for Reforestation.
An ancient form to harvest water is by making a body of water or pond. Once the pond is dug, and the pond surface is saturated with water the water level starts to rise. And after some time a huge amount of water can be saved. Nonetheless, a common pitfall of the system is the loss due to absorption through the ground and evaporation. However, modern approaches use geomembranes or some other waterproofing techniques to prevent absorption loss. The following exemplifies a method to determine the size of an insulated water pond fed by a small rainwater harvesting system that provides water to a small reforestation operation.
The reforestation operation consists of a small tool shed with a rainwater harvesting system with a surface area of 16 square meters and about twenty small pine trees planted in plastic bags. After a few months when the trees are big enough the trees are planted in the surrounding area. Once the trees are in its final location the trees are irrigated for a few weeks to ensure its survival. The daily demand for such operation is about 50 liters per day or 1500 litters per month.
The data
The data consists of about ten years of climatological records, although the data is outdated, it represents the best approximation available. To analyze the available data three different scenarios will be defined. The max scenario consists of the maximum data value of a given day, the min scenario consists of the minimum value of a given day, and the mean scenario consists of the mean value of a given day.
Daily rainwater data ranges from zero to eighty millimeters of waters per day, and the rain season has a duration of about one-third of the year. Whereas evaporation data appears to be more consistent throughout the year. Ranging from zero to ten millimeters of evaporation, however the mean evaporation values plateau in two millimeters approximately. Both evaporation and precipitation data display a flat line on zero on the min scenario, thus the min scenario will not be discussed any further.
Net available water per day stays on the positive side for about half of the year, while the negative side appears to be concentrated on the first months of the year. The same is true for most of the different historical scenarios. Taking into account only the mean and max scenarios, the total accumulated water available through the year result in two extreme approximations. Under the mean scenario, there is bearly enough water to be harvested, while the max scenario results in an available harvest of about 3000 millimeters
To determinate the size of the pond we use the max scenario, the basic idea is to subtract the daily demand and to add the available rainwater. Then the number of days with water for the operations is calculated with the following.
Under the max scenario, precipitation alone can provide enough water for about 200 days. And to provide enough water for the remaining of the operation a reserve pond of about 8000 liters is needed.
The max scenario provides an extreme scenario that could be unlikely to happen, which can be observed in the huge difference between the mean and max scenario for the net available rainwater. To take the variability into account a different approach to determine the size of the pond will be taken. First, a successful size will be defined as the size needed to provide water for the entire year. Then, a random precipitation and evaporation profile is generated, using as limits the min and max scenario. And for each random profile, the success ratio is calculated. The success ratio for this simulation is defined as the ratio between successful simulations and the total number of simulations.
Stochastic simulation results in an increase in the pond size needed to provide enough water for the reforestation operation. A pond of 10000 liters is needed to provide enough water for the reforestation operation. The increase in the size could be the result of an increase in the evaporation values in the simulations, a decrease in the precipitation values, or a combination of both.
Know you know how to analyze historical climate records, discrete and stochastic simulations to determine the size of a small pond for a small reforestation operation. The complete code for this post can be found on my Github by clicking here, and the dataset by clicking here. See you in the next one.
