Characteristic of the Regional Rainy Season Onset over Vietnam: Tailoring to Agricultural Application

Abstract

Owing to its unique position within multiple monsoon regimes, latitudinal extent, and complex topography, Vietnam is divided into seven agroclimatic zones, each with distinct rainy season characteristics. Variation in the dominant rainfall system across zones affects the rainfall climatology, the primary water resource for regional crops. This study explores the creation of an agronomic rainy season onset based on high-resolution rainfall data for each agroclimatic zone for applications in an agricultural context. Onset information has huge practical importance for both agriculture and the economy. The spatiotemporal characteristics of zonal onset date are analyzed using integrated approaches of spatial and interannual variability, temporal changes, and estimation of predictability using teleconnection with Niño 3.4 sea surface temperature anomalies (SSTA) for 1980 to 2010. Results suggest that northern and southern zones experience regional onset dates in May, while the central zones experience rainy season onset in late August. The regional variability of rainy season onset is lower in the northern and southern zones and higher in the central zones which are latitudinally extended. The interannual variation in rainy season onset date is found to be approximately two weeks across all agroclimatic zones. The significant negative trend in rainy season onset date is found for Central Coast and South Central Coast zones, suggesting that the onset date shifted earlier for the entire period. In the decadal scale, the zonal mean onset date shifted later in the Northwest zone and earlier in the Central Highlands. Out of the seven climate zones, a significant positive correlation is only noticed in the Central Highlands and South zones between zonal mean onset date and Niño 3.4 SSTA for Dec–Jan–Feb, suggesting the potential of seasonal scale predictability of rainy season onset date with respect to preceding El Niño-Southern Oscillation (ENSO) events.

Local‐scale rainy season onset detection: A new approach based on principal component analysis and its application to Vietnam

“…The main goal of this study was to propose a new approach to define suitable rainfall-based criteria at the local scale and its applications to 131 stations in Vietnam. Previous studies have used several rainfallbased criteria to determine RSODs for different areas across the world including Vietnam (e.g., Stern et al, 1981;Sivakumar, 1988;Omotosho, 1992;Moron et al, 2009;Nguyen-Le et al, 2014;Ngo-Thanh et al, 2018;Pham-Thanh et al, 2019;Acharya and Bennett, 2021). Each study has its own advantages and disadvantages, in which either local-scale (i.e., stationbased or gridded rainfall) or regional-scale (i.e., areal average-based rainfall) information were used to generate the RSOD criteria irrespective of using subjective or objective approaches, leading to pronounced differences of the determined RSODs.…”

“…The five consecutive days (length of the first wet spell of rainy season) in (a) and seven consecutive dry days (maximum length of a dry spell occurring after the RSODs) in (c) were fixed based on previous studies (Stern et al, 1981;Moron et al, 2009;Ngo-Thanh et al, 2018;Pham-Thanh et al, 2019;Acharya and Bennett, 2021). Thus, the detected RSODs are only based on the thresholds of P, N, and D. However, the set of the optimal thresholds is not useful outside of the region for which they were developed.…”

Variability of rainy season onsets over East Africa

“…Forecasts of the timing of the seasonal rain onset are often ranked by users as the most important metric (Owusu et al, 2017; UK Met Office, 2011). Accurate knowledge of the onset date can boost yield by reducing the risk of planting late or early (Laux et al, 2008) by guiding the implementation of key cropping decisions such as land preparation, purchase of inputs, sowing dates, mobilization of manpower and equipment (Acharya & Bennett, 2021). For example, the delayed onset of March‐May (MAM) rains in 2019 was associated with a 20% reduction in total maize production in Kenya (Global Agricultural Monitoring, 2019).…”

“…While onset can be defined using a combination of atmospheric circulation parameters, most definitions are based on rainfall only, due to the availability of historical rainfall data—compared to other parameters such as soil moisture—and the direct relevance to agricultural practices (Ferijal et al, 2022). “Agronomic” onset definitions are typically based on point‐scale rainfall exceeding some threshold of accumulated rainfall, deemed sufficient for seed germination but with an added condition to avoid false onset, that is, no significant dry spell in the subsequent days (Acharya & Bennett, 2021; Laux et al, 2008; Marteau et al, 2009; Stern et al, 1981). Forecasting centres in EA such as the IGAD Climate Prediction and Applications Centre (ICPAC) operationally utilize agronomic definitions (Table 1) across the entire region/country.…”

Emerging Hydro-Climatic Patterns, Teleconnections, and Extreme Events in Changing World at Different Timescales

“…In addition, precipitation over the HRB was triggered mainly by the vapor transport from the west to the east. Nachiketa Acharya [5] explored the onset of the rainy season for seven agroclimatic zones over Vietnam. The spatiotemporal characteristics of zonal onset date are analyzed using the teleconnection with Niño 3.4 anomalies.…”