Implications Of Unusual Spring Rainfall Pattern

Media, when reporting weather updates should focus more on characteristics of rainfall rather than the daily sum. This issue is extremely important to educate the public to avoid harmful impacts and reap the benefits of preferred characteristics. 

The drought breaking rains: Nepal received unprecedented characteristics of rainfalls in the second half of March, 2024 after a prolonged winter drought since the departure of monsoon in October 2023. Initially two cyclones that simultaneously emerged from the Bay of Bengal and Arabian Sea produced significant quantity of rains in a pattern of high to low volumes from the southern to northern ecozone.

 In the subsequent week, powerful impulses from the westerly winds brought more moistures into the region to produce additional rains, thus, exceeding the normal quota of the season. These rains restored profound hopes and positivity in the deteriorating environment of the Himalayan region and the densely populated downstream region. The purpose of this column, however, is to highlight the importance of focusing on the shifting rainfall characteristics instead of rainfall volumes. 

Unusual characteristics: These characteristics of rains are rare phenomenon in Spring season when gusty winds, thunderstorm, hailstones and short yet intense showers are the usual characteristics. None of the rainfall events with these characteristics were noticed this time. Several days of calm and continuous rains with low to moderate intensity are the typical characteristics of mid monsoon weather (i.e. August). In other term, these types of rains are popularly known as ‘Jhari’ and, has two types- Barkhe Jhari (occurring in rainy season) and Hiunde Jhari (in winter) indicating cloudy weather with sustained rainfalls for several days (usually 3 to 7 days).  

Analyses of historical rainfall data show that these characteristics are in decline and short, intense and localised rainfalls, also known as cloudbursts, are on rise across the country. In this context, the rains that we experienced this time in March, are exceptionally beneficial to our economy and environment. 

The blessing showers: These rains brought a range of spectacular benefits. The showers doused the razing forest fires that were spreading all over the country and flushed out the polluting air particles to restore clean air. They restored the snow cover all over the high mountains and rejuvenated the flow in the drying spring sources. They restored moistures in the parched lands, allowing to cover them with vegetation in few weeks of time.  These rains saved the topsoil from erosion because of low intensity of rains initially when the soil was open without vegetation as well as restored the depleting groundwater reserve, though partially. They have regained hopes that not all changes in the climate system are harmful. 

Rainfall characteristic matters over volume: The rainfall data often shows the volume or total amount of rainfall received in the last 24 hours or any specified duration. The data, however, doesn’t reveal whether the quantity of rain occurred in few minutes, hours or uniformly over 24 hours. An hourly or half an hourly period of rainfall data, instead, reveal true characteristics of the rains as those data reveal the nature of impacts on the surroundings. For example, 50 millimetres (mm) of daily rainfall has been reported, and, the details are available on hourly basis, then this total volume could be the result of several hours of rainfall not exceeding the intensity of 10 millimetres per hour over the 24 hours. This pattern of rainfall can be assumed highly beneficial to crops and vegetation having no strong current of runoff and soil erosion. On the flipside, if the same 50 millimetres of rain is received within an hour or less, then one can assume its destructive characteristic due to its capacity to generate strong currents of runoff leading to soil erosion, flashflood and congestion of drainage systems. 

However, the major significance of rainfall characteristic lies on its capacity to recharge groundwater system that occurs in the range of a few mm to a few cm per hour, usually less than 10 cm per hour, depending upon the nature of land surface and compactness of soil. Therefore, any rainfalls of high intensity, typically that of 25 millimetres per hour or more, often ends up generating high runoff and flooding with minimal contribution to the groundwater reserve. Because of Nepal’s mountain topography, these characteristics are synonymous with less beneficial and more harmful. This is particularly true during the spring season when most of soils remain open and loose susceptible to erosion.

In conclusion, the shift of rainfall characteristics from impulsive (intense and harmful) to rhythmic (calm and sustained with low to moderate intensity) is a rare positive sign noticed from the pattern of spring rainfall this year. Tracking of rainfall characteristic is crucial to develop response mechanism particularly to save water resources and agricultural businesses. 

 (Dahal writes about water, climate and environment and is associated with Nepal Water Conservation Foundation Kathmandu. ngamindra@gmail.com)