Dear Mekonnen Gebru,

The method I suggest is to use the Maize growing degree method and extrapolate it to you tomato situation.

If water and nutrients are all available the advancement of crop stages is a function of accumulation of heat which are measured in units above a minimum for growth which 10 C for maize and to 30 C which is the growth maximum. The daily high temperature up to 30 C is substracted from the low temperature and it is averaged and that result is taken minus the 10 C minimum temperature. The results are tallied day by day into cummulated heat units. 

These are were calculated in Maize models. I would suggest you take the phenological stages as described in the literature and tally the growing degree data you collect this will allow you to readily understand and predict when different crop stages will occur.

Your crop requirement can be measured according to the different stages and the needs for example for irrigation can be determined with precision. These responses can be confirmed by plant and soil sensors. 

Good results on your endeavor. Paul Reed Hepperly, Scientist 

Measure number of days for each development stages depend on the crop type, and each development stage has some aspects should be appear in the field for more than 50% of plants

I hope it is to understand first about the peculiar characteristics (phenotypic markers) of the different growth stages. After knowing these descriptors, it is very easy to determine growth stages when the plants reached at 50% of that pecular characterstics determining the grwoth stages. Some researchers use phenological characters i.e days to reach 50% of the different growth stages. E.g 50% of shoot initiation, Days to 50% first flowering, days to 50% shoot elongation, days to full flowering, days to emergence, days to end maturity etc.

Dear Mekonnen:

You are correct in that different stages need different amounts of water  ... and all water stage needs are influenced by crop; type, humidity, wind, temperature, rain fall, soil type, amount of light etc. ... which we monitored and used the Blaney–Criddle equation in a community garden as a relatively simplistic method for calculating evapotranspiration and thus watering schedule but we guessed at the stage of growth by dividing the growth period into four periods ... you are taking an interesting approach ... I do not know of anything that is simple yet accurate.

Mr.Mekonnen,good question and four interesting comments.For me 50%  of each stage development is appealing for undertaking different nutritional, watering,and pest control measures.For many crops the growth stages are fixed.The stages are available in pictures,table and chart form from different sources..Please see for example the images for tomato plant growth stages/charts on internet.However the stages may vary in days from variety to variety.So one has to see the distinguishing features of each stage and decide 50% of that particular stage before undertaking a  crop management operation.

I am enclosing some good examples of the phenological stages of tomato.

Calculating accumulated heat hours above base temperature (12°C).