Kent County Council

Rainwater Harvest Calculator

Web Application


2022 - 2023 (Ongoing)


London, UK


Due to the challenges posed by climate change, a growing population and changes in patterns of land use, Kent’s local water systems are under significant pressure. In response Kent County Council in partnership with Cranfield University had developed a rain water harvest calculator in the form of an Excel tool.

They needed this to be converted into a clear and easy to understand web application, to help farmers work out rainwater harvest potential for their farm.


Japeto, formerly known as Fintek Software, took on the task of creating a user-friendly rainwater harvesting calculator tailored to the needs of farmers. We utilised the existing Microsoft Excel tool, enhancing it with the necessary data sets and algorithms, and transformed it into a straightforward and visually appealing online platform. This web-based tool now seamlessly integrates into our website using an iframe.

The improved tool provides access to 30 years of historical rainfall data, making it easy for polytunnel fruit farmers to calculate their irrigation water needs from rainfall, eliminating the expense of external sources. By simply inputting their irrigation and polytunnel covering dates, the calculator generates accurate rainwater accumulation predictions for various scenarios, including wet, average, and dry years. Armed with this valuable information and an understanding of their crop’s irrigation requirements, farmers can make informed decisions to save on water costs.

Live Data

The tool’s purpose is to explore how different sizes of a rainwater harvesting system affects cost savings. In the Excel tool, this takes time: you enter a single size and look at how it would perform. There’s no way to directly compare different sizes.Our tool addressed this by providing tools for farmers to adjust the size of the rainwater harvesting system and see the results updating in real-time. The results page lets the user change the size with a simple slider, and the page changes to show how that size of rainwater harvesting system would perform.


Local governments are legally required to meet a certain standard of accessibility so that people with disabilities can use their online services (WCAG 2.1 AA standard).We built and tested the calculator to comply with this standard. This involved using manual and automated testing to identify issues with colour scheme and contrast, (for people with visual impairments), making sure text-based alternatives exist for all graphics and graphs, and ensuring the structure of the calculator worked with screen readers and other assistive technologies.We provided an accessibility report which could help KCC ensure they comply with accessibility rules.


Ease of Use

The original tool was powerful but could be somewhat confusing to set up and use: it needs the user to have Excel installed and the user needs to change their security settings to run the tool. The data entry is quite overwhelming and the tool has an in-depth user manual.Our calculator addresses these issues by being easily accessible via the web. We made sure that all the data input fields had clear explanations through labels and tooltips, and also worked with KCC to identify the data inputs which don’t really change – we moved these to an “advanced” section which is hidden by default, to simplify data entry for most users.

Understanding the Tool

The Excel tool contains complex logic for calculating rainwater harvesting based on many factors, such as historic rainfall data, the location and size of the farm, how different crop types use water, and sowing / harvesting times.We worked with KCC to understand the Excel model and how soft fruit farmers operate, and reverse engineered the Excel algorithm to reproduce it in the calculator. Once we had a working solution, we tested the calculator comprehensively by creating tools to directly compare the results from the Excel tool with our model in a wide range of data inputs.The result is that our calculator provides identical results, but with a presentation which is much more user-friendly.


Farmers in Kent can use the Rainwater Harvest Calculator to determine how much rainwater they need to harvest for their crop and how this will equate to financial savings.


Everything’s been done on time or early. All of our feedback has been promptly incorporated.

Tom, Water Resource Manager at Kent County Council