Trend analysis of temperature, relative humidity and the equilibrium moisture content of wood over the Last Half-Century in Iran

Introduction

Equilibrium moisture content definition (EMC) is the moisture level where the wood neither gains nor loses moisture since it is at equilibrium with the temperature and relative humidity of the surrounding environment. Climate change has been one of the challenges in recent century and has affected the sectors related to it. One of the most important and influential parameters affected by these changes is the Equilibrium moisture content, which is of special importance in the sciences related to wood industries and their products. EMC is the moisture level where the wood neither gains nor loses moisture since it is at equilibrium with the temperature and relative humidity of the surrounding environment. The EMC of wood is strongly influenced not only by the physical properties of wood but also by temperature and relative humidity. The relative humidity in air varies with the temperature and the amount of water damp present. The hotter the air the more water damp it can hold. The EMC will follow the RH in air, through a direct relationship if the wood and air left to reach equilibrium. Therefore, changes in these two climate variables will affect the EMC of wood and consequently alter the physical properties of wood products. The aim of this study is to investigate the trends of temperature, relative humidity, and the EMC during the period of 1961-2015 at 27 selected stations in Iran.

Materials and Methods

Calculating the EMC based on the relative humidity and temperature

Although the relationship between RH and EMC of wood is not linear, an increase in RH or a decrease in temperature will increase the predicted moisture content of wood, after its equilibration with the air. The non-linear nature of the RH-EMC relationship is a typical sorption isotherm and has been described by sorption theory. An adsorption model employed by Simpson (1973) uses the theory developed by Hailwood and Horrobin (1946) to predict the EMC based on the combination of temperature and RH. The form of the equation is as seen in Eq. 1,



(1)

Where EMC is the equilibrium moisture content (%), h is the relative humidity expressed in decimal form (% / 100), and W, K, K1, and K2 are coefficients defined by Eqs. 2 through 5, respectively,



(2)

(3)

(4)

(5)



In this equations T is the dry-bulb temperature (°F).Thus, given two pieces of information, dry-bulb (or ambient) temperature and the RH, the EMC can be readily calculated.

The Mann-Kendall Test



The purpose of the Mann-Kendall (MK) test (Mann 1945, Kendall 1975, Gilbert 1987) is to statistically assess if there is a monotonic upward or downward trend of the variable of interest over time. It does not require that the data be normally distributed or linear. It does require that there is no autocorrelation.

List the data in the order in which they were collected over time, x_1, x_2, …, x_n,which denote the measurements obtained at times 1,2,…,n, respectively, the MK test statistics is calculated based the sign of the difference between two consecutive observations, 𝑠𝑔𝑛(𝑥𝑗 − 𝑥𝑖).

The sign function is

(𝑥) = 1 𝑖𝑓 𝑥 > 0; = 0 𝑖𝑓 𝑥 = 0; = −1 𝑖𝑓 𝑥 < 0 (1)

and then

𝑀 = ∑ (𝑥𝑗 − 𝑥𝑖)1≤𝑖≤𝑗≤𝑛 (2)

Under the null hypothesis (𝐻0) of no monotonic trend, M has asymptotically normal

distribution with mean zero and approximated variance:

(𝑀) = (𝑛 − 1)(2𝑛 − 5)/18 (3)





Results

The results of this study, conducted using the non-parametric Mann-Kendall method (monthly, seasonal, and annual time scales), showed an increasing trend in annual temperature at 25 stations and a decreasing trend in annual relative humidity at 18 stations. The annual trend in EMC also had a significant decreasing trend at 20 stations. The result indicated that the EMC over a 55-year period is influenced by changes in temperature and relative humidity, such that the increasing trend in temperature and decreasing trend in relative humidity have led to a decrease in the EMC percentage.



Discussion

Over the past decades, there has been a significant change in climate which has increased the attention towards the potential effects of the changes in different sectors including timber structures. From various research on wood and its behavior and characteristics, it is known that wood is very sensitive to changes in climate, shown especially by shrinkage and swelling, but also by properties like stiffness and strength (Lanata, 2014). According to these results, it was determined that the trend of the EMC in the 55-year period were affected by the trends of temperature and relative humidity. These results should be considered as a warning to the timber industry and similar industries that are affected by the climate variability and climate change.



KeyWords: Trend Analysis, Relative Humidity, Temperature, Man-Kendall, EMC