Practically speaking , Sunnhemp does not need so much water ..It belongs to leguminaceae plant family like black gram, green gram, beans etc ..Excess water is detrimental to the plant growth ..
For first two weeks light soil moisture is enough for germination ..This moisture can be provided either by rainfall naturally or soil moisture present after harvest of paddy in river delta region or light irrigation..
Next irrigation can be given 3 week later or a month time depending upon your soil ..If your soil is clay or loam you may give subsequent irrigation a month later .
If your soil is sandy or gravel you may need to give subsequent irrigation in about fortnight time ..
Of course any plant needs water for its growth and development .No plants grow without water .
In this case of sunnhemp green manure crop it is minimum soil moisture that suffice
Regarding cost of sunnhemp cultivation it is as follows
Seed rate per acre - 10 kgs in case of good quality seeds or 15 kgs in case seeds are poor in germination ..
Seed cost Rs.100 / kg for good quality and Rs.80 for poor quality
Only seed and ploughing costs are counted here
In our case we could not irrigate the sunhemp field ..we relied on rain water and our soil is sandy gravel with poor water holding capacity
However we did sow the sunhemp seeds 3 weeks before recent rain ..That is we anticipated summer shower that is usually expected to be around 100 mm that is enough for germination and early growth of any plants ..Unfortunately we did not receive any summer shower ..instead hot sun with mercury level around 40 degree celsius prevailed .3 weeks after sowing all the sunhemp sprouted well on account of recent hail storm at our site ..we are not watering sunnhemp ..
we are waiting for rain ..if no rain in next 10 days we will resort to spraying water by hose pipe ..