#52 - Aug 2021 - Garden Stuff - © - Sandy Lang - slang@xtra.co.nz
HYDROLOGYAugust: Late winter. Get on with the pruning… A record warm June. No good for fruit trees needing winter chill for flowering. Hopefully enough cold in July/August? Google chill requirement Four seasons: The Earth’s tilt and yearly orbit round the sun determine the annual solar cycle – the equinoxes and solstices. Add in a three-week ‘temperature lag’ and these determine the middles of the four seasons - spring (Sep, Oct, Nov), summer (Dec, Jan, Feb), autumn (Mar, Apr, May) and winter (Jun, Jul, Aug). Plant water use: A forest or pasture or crop uses water at a calculable rate that depends on the incoming sunlight energy (daylength, light intensity) and the air (temperature, wind, humidity). Scientists talk about “Penman–Monteith potential evapotranspiration” = ‘PET’ for short = estimated plant water use (pretty accurate). Google potential evaporation – read to get the feel – just ignore the maths…! Soil water balance: For Wellington, annual rainfall (926 mm) and annual water use by plants (866 mm) are about in balance. In spring, rainfall (236 mm) and plant water use (250 mm) are also about in balance and the same for autumn rainfall (222 mm) and plant water use (165 mm). However, in summer, rainfall (186 mm) is only about half plant water use (380 mm) while in winter, rainfall (282 mm) is about four-times plant water use (71 mm). So, summer is the season when soil water reserves are run down and winter the season when they are replenished. These rainfall and plant water use values are long-term average ones, the values in any particular season vary +/- quite widely. Losses: But not all rainfall enters the soil+plant system. (1) About 3 mm of every rainfall event is lost. It sits as drops on the outside of the leaves, later evaporating. It enters neither the plant nor the soil. (2) Heavy rain lands on the ground faster than it can infiltrate, so some is lost, it runs off to the sea in creeks and rivers. Most places in Wellington are a bit dry, most of the year. So, some irrigation is needed in spring and autumn and more in summer. Many places on Earth have lower rainfall and higher plant water use than Wellington. How do forests cope? Teamwork and hydraulic redistribution help – but that’s for next month…!___________________________________
HYDROLOGYAugust: Late winter. Get on with the pruning… A record warm June. No good for fruit trees needing winter chill for flowering. Hopefully enough cold in July/August? Google chill requirement Four seasons: The Earth’s tilt and yearly orbit round the sun determine the annual solar cycle – the equinoxes and solstices. Add in a three-week ‘temperature lag’ and these determine the middles of the four seasons - spring (Sep, Oct, Nov), summer (Dec, Jan, Feb), autumn (Mar, Apr, May) and winter (Jun, Jul, Aug). Plant water use: A forest or pasture or crop uses water at a calculable rate that depends on the incoming sunlight energy (daylength, light intensity) and the air (temperature, wind, humidity). Scientists talk about “Penman–Monteith potential evapotranspiration” = ‘PET’ for short = estimated plant water use (pretty accurate). Google potential evaporation – read to get the feel – just ignore the maths…! Soil water balance: For Wellington, annual rainfall (926 mm) and annual water use by plants (866 mm) are about in balance. In spring, rainfall (236 mm) and plant water use (250 mm) are also about in balance and the same for autumn rainfall (222 mm) and plant water use (165 mm). However, in summer, rainfall (186 mm) is only about half plant water use (380 mm) while in winter, rainfall (282 mm) is about four-times plant water use (71 mm). So, summer is the season when soil water reserves are run down and winter the season when they are replenished. These rainfall and plant water use values are long-term average ones, the values in any particular season vary +/- quite widely. Losses: But not all rainfall enters the soil+plant system. (1) About 3 mm of every rainfall event is lost. It sits as drops on the outside of the leaves, later evaporating. It enters neither the plant nor the soil. (2) Heavy rain lands on the ground faster than it can infiltrate, so some is lost, it runs off to the sea in creeks and rivers. Most places in Wellington are a bit dry, most of the year. So, some irrigation is needed in spring and autumn and more in summer. Many places on Earth have lower rainfall and higher plant water use than Wellington. How do forests cope? Teamwork and hydraulic redistribution help – but that’s for next month…!___________________________________